I got tired of swapping my speedplays to spd's so I just use A520's now on the gravel bike.

A520 or A600 is a great road/gravel pedal. Using MTB shoes that are on the stiffer side will make performance indistinguishable from a road shoe/pedal combo, the only drawback is MTB shoes & pedals are slightly heavier.

The biggest difference you will notice is the stiffness of the shoe sole. Road shoes are crazy stiff and only the top line mountain shoes are remotely that stiff.

When I ride my cross bike on group rides I can feel the shoe flex around the cleat mounting plate.

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I'm an Crank Bros eggbeater guy and use them on all my bikes (S-Works Epic, Air RDO, Hakka MX, S5, and even my P5). I use the S-Works MTB shoes and they're reasonably light, quite stiff, and allow me to walk easily if I'm out training, want to stop and hang out, fix a mechanical, etc... I run the 4ti pedals, so they're quite light too. And for muddy gravel/CX stuff I really like the mud clearance. They were great in the Paris-Ancaster mud...

Other than in a "real" race on the P5 or S5, I don't feel that I lose enough in power transfer, etc to justify giving up on the convenience of the MTB shoes and pedals. YMMV.

Buy another set of zeros to leave on the SuperX when it's in "road mode". Always a set for sale in the classifieds here.
I learned long ago that when you have multiple bikes and single sets of pedals, switching is fast but a hassle.
Not sure why you would want to ride the SuperX on the road over the R3 though.

I would actually do the opposite and buy a pair of XTRs and leave them on the SuperX. But I do agree that if you have multiple bikes, you should have one set of pedals for each.

OP - You can still take the SuperX out on road rides with your MTB shoes, but if you're ever in a spot where you're worried about "giving up advantages" due to pedals and shoes...then your answer is pretty simple. Use the R3.

If you're saying that you are turning the SuperX into a full time road rig, then sure, get speedplays and call it a day, but my assumption is that you still want to take it off road.

You're not giving up anything with the XTR pedals. I used them for a while on my TT bike when I custom drilled some shoes for mid-foot cleat mounting. Personally I'm a big fan of the Speedplay Zeros but SPD pedals are perfectly functional and to tease out any sort of "performance" advantage/disadvantage would be quite an undertaking (read: if there is one it's very very small).

SuperX is a great bike too, love the geometry of the new one. I kind of hope they make one with the new Lefty with perhaps 60mm of travel.

The difference is either so small that nobody has managed to measure it, or so small that marketing teams have chosen not to use it as a sales argument.

I use "road" pedals on most of my bikes, but I've never gotten the sense that I'm actually giving up anything tangible with SPD. If feel or appearances are a concern, go with A520 or A600; they look like road pedals and can make wider contact with the shoe sole than most other SPD varieties. I use A520 on my gravel bike and love them.

Thanks for all the good info everyone. Much appreciated.

I think you are giving up zero if you have a stiff and light pair of mountain bike shoes.
I use Bont for road and offroad- The Bont Vapor G is essentially their road shoe, but with drilling for a 2 hole cleat and a bit of tread.
Specialized S-works Recon is similar in that regard.

I occasionally ride my gravel bike (Salsa Warbird) on road group rides with spd’s and my mtb shoes. I usually do it when the weather is sloppy, roads are wet from overnight rain or recently when my road bike is already packed up for a trip. Other then high efforts out of corners I haven’t noticed any issues.

I used egg beaters on my gravel/cross bike, which I actually end up riding more of these days b/c I'm so concerned about stupid drivers on the road. I run Fizik Terra x5 shoes and everything feels great. I thought I'd get hotspots because of the smaller pedal, but nope. Super comfortable and the bike flies on climbs.

I'm loving my A600 pedals on my Groad bike. Have done group road rides, group gravel rides, gravel races...works pretty slick. I use my Giro Empire MTB shoes and really like the setup. Have considered going that route on my tri bike feeling like I can probably run faster in transition with the Empires on that I can pansieazzin' around barefoot or clippity clopping it in the road shoes.

I started to get some knee pain with my Shimano XT pedals on. Moved to Time ATAC, may not have been the pedals though. Certainly won't impact performance.

I've done this for years. Shimano MTB pedals are what I use (the one below XT... I think it's called the 520 or 540... basically Deore/SLX level. Inexpensive, and they work). Plus MTB shoes are much better for walking around in. It's probably what I'd do full-time now, if it wasn't for the fact that I've had a bunch of shoes coming in to test/review.

I dont anymore but I did for awhile... my gravel bike had XTR9000 and now xpedo CXR pro pedals and I used to use it as my road bike until I recently N+1'd.

I certainly would not hesitate to use them again, the shoes are a hell of alot easier to walk around in. I actually plan to get my wife a pair of MTB shoes and the XTR9000 pedals will go on her road bike.