Is there any hard data as to if larger platforms (Shimano, Look) transfer more power than smaller (Speedplay)? If not, any anecdotal evidence?
I think that is a great question. I don’t have the answer, but I wonder if anyone else does? I’m not sure if this is as on topic as “movies you can watch while on the trainer,” however, so perhaps you will not receive an answer.
I also wonder whether anyone has thoughts on your other previously unanswered question: is the $35 premium worth it to step up from Speedplay Zero Chrome-Moly to Stainless? Also, do people find Speedplay Zeros easy/average/heard to get into/out of? Thanks!
If not, any anecdotal evidence?
Anecdotally, I say it’s bunk. I have speedplay pedals, and I can’t tell how big or small the pedal platform is. Seems to me that the stiff soles of bike shoes function as the platform.
I agree with Vitus. With modern cycling shoes, particularly $300+ carbon soled shoes, you should be able to pedal on a pea and not notice the difference.
As for Speedplay Zeros, I would get the stainless just out of vanity. With the older X series, they used different bearings on the CroMo version, but they appear to be the same on the Zeros.
While Speed isn’t really my main concern, I use a several versions of double sided MTB SPD pedals.
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They are cheap
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They’re way easier to get into when leaving T1 than just about anything (except may eggbeaters) It’s so easy getting into them that I find it faster to put them on in T1 that I put my shoes on and run through transition and leap to my stead, knowing I won’t be wobbling for 50 metres to get them in.
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Metal cleats take years to wear out based on the fact that I use them to commute to work every day and haven’t had to replace a pair in 3 years.
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With carbon soled shoes, the platform size isn’t an issue
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The have adjustable tension (waaaaay easy to adjust) and have varying amounts of float.
All round great solution to the problem of shoe/pedal interface.
So many things are so untrue.
I have ridden them all - and I tell you what, I can feel the Time Titan Mag pedal is not the same as my new Time RSX Carbon Ti pedal, I can “feel” that the new pedal is smaller. This is with >$300.00 carbon sole shoes in fact, with the same shoe. I dont care what you pay for your shoes, there is some flex in the sole - the new Sidi shoes are even being made with some flex in them. If this is not an issue…why would ShimaNO redesign their pedals 3 times in 2 years (and go to a large platform)?
Part of the reason that Shimano SPD’s seem to last a long time is that they are most often used with MTN shoes and the cleat itself is protected from contacting the ground.
Ease of adjustment? I can not find a better pedal than the OLD Time pedals - there was no adjustment. And I dont know anyone who wanted to adjust them. Put your foot on their sweet deal that would tell you what setback number to put your cleat at - mount the cleat at that number (on your Time shoes) - done.
I use SPD with MTN shoes for commuting and also with some ShimaNO 
carbon shoes from '99 I think.
The cleats on the MTN shoes are a little shiny on the bottom and that’s about it. As you say, they are pretty well protected by the shoe.
The cleats on my road shoes have spent many miles clomping around cafes as well as normal mal treatment that bike shoes get. I have replaced the cleats once (since '99), when I got some new SPD pedas a few years ago, simply because I assumed the old cleats would be worn and a new set came with the pedals. When I got the old ones off (not an easy task, with rusted bolts) I couldn’t see significant wear between the old cleats and new cleats but I changed them anyway.
As for adjustment I have my commuting pedals set relativley loose since I stop at stop lights. (we ALL should) and I like to clip out easily. I also like to be prepared for when some bozo driver knocks me off.I don’t want my pedals too tight, so I can release quickly, without additional injury to my knees from being trapped in my pedals.
My road pedals are set tighter because I get out of them less often, since long rides are out inthe country. The risk of a vehicle accident is hopefully less so it’s not as big an issue. I also like to be held in tighter when climbing. I’m FAT, so grinding up hills puts a lot of strain on the pedals. If they were set too loose, I would increase my risk of inadvertantly releasing when I didn’t want. Particularly when standing, since I use all my mass, moving around alot, to get my bulk up those damned hills 
Easy adjustment is good.
I would venture to say that transfer of power will be exactly the same in any sized pedal, whether it is 4 inches square, or ‘the size of a pea’.
The variations that may occur would be pressure, shoe deformation, and comfort.
Assuming that you are pushing your pedal with x amount of force, and with two pedal sizes being the only variation; there must be energy displaced somewhere besides the pedal in order for there to be a loss in transfer of energy. For example, if your pedals were 12" square, then a small amount of energy would be lost to friction and air pressure, thus less energy transferred to the cranks.
With equal force applied to the two distinct pedals, the pressure would increase as the area of the pedal decreased. The small change in area would probably be negligible, especially considering the fact that most shoes are hard soled enough that the increased pressure would still be distributed over the surface are of the shoe.
That being said, no shoe is 100% rigid, and with a very small pedal, some energy will be lost in the shoe as it deforms around the pedal. Imagine, as an example, pedalling a bike with speedplay pedals, wearing flip-flops. the flip-flops would bend quite a bit around the pedal = lost force. It could be argued that this lost force isn’t lost as most shoes with plastic or carbon soles would rebound the foot as the shoes returned to its neutral state, creating a reciprocal upwards force on the foot.
Comfort, though unquantifiable, may be distinct on different sized pedals. Most comfort differences would probably be a result of pressure on a softer soled shoe.
You should probably be more concerned about time to enter the pedal, float or lateral give, and weight more than lost force.
The zeros aren’t hard to get into unless your cleats are worn. IMO. Takes minimal force. Compared to the pedals of some of the guys I ride with, I would even classify them as easy. I’m always in first.
How about opinions on the old shimano spd pedals being ‘loose’? What I mean is there is a lot of slop laterally, like if you try twisting the shoe (while clipped onto the pedal) from right to left, etc. (Not float). I wonder how this affects pedaling efficiency and whatever else?