Smarter people than me will respond but will give my thoughts based on what I have heard here on ST.

On the tri or TT bike, crank length is a way to influence hip angle and balance the aero of the stack in your position with power robbing hip angle.

Road bike? You seem sitting up high enough it shouldn’t matter at all. Maybe for a Merckx road bike TT person or a solo breakaway specialist. I say for road bike just ride what feels best to you. If you are short, maybe a shorter one. Average height like most folks, keep the one you have. Super tall, maybe a longer one.

Cliff notes is that the advantage of a short crank in TT doesn’t seem to be relevant in normal road positions assuming your fit is good for your body (height, inseam, etc).

How tall are you?

I'd agree. The evidence I've read suggests a wide range of crank lengths don't make much difference to power. I did find 165mm cranks on my cross bike did feel weird.
In general going shorter has few downsides but going too long can be a problem.

6'1".
Bike is a 58, Trek's H2 geometry. Cranks are currently 175.

At your height, I would stick with 175mm on a road bike.

No reason to spend money.

Okay, thank you!
For my own edification, are there any positives or negatives to decreasing to 172.5 or 170?

If you switch to a shorter crank, you lose a bit of leverage.

6' here... ride 175 on all my bikes since forever. I never really bought into the shorter crank movement

I'm 6'-0" and sized down my road bike frame size to 56cm with 72.5 cranks. For me it is a much better fit than the 58cm / 175 scenario.

My TT bike is a Trek SC (large) with 165 cranks.

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"They know f_ck-all over at Slowtwitch"
- Lionel Sanders

... but gain the ability to hold a slightly higher cadence. (Not suggesting either is better - just the trade off)

If I'm buying a whole new groupset, is it worth $15 extra bucks to stay with the 175s?

I recently went from 175mm to 170mm. This would be considered a short crank size for my endurance geometry bike and my height (6'2"), but I wanted to experiment and see what it would feel like.

I actually prefer the shorter cranks, and haven't thought about sizing back up. I feel like I have more control over my pedal stroke. And for what it's worth, I haven't been able to discern a decrease in my performance.

That's my N=1.

I think you are saying that 172.5mm cranks would be $15 less than 175mm.

If that is the case, I would go with 172.5mm.