[Nonfractal]

Jeremy,
You will find lots of discussion and disagreement on this subject. 

For me, the base assumption you make isn't right. 
Gearing is a factor or Torque, not of RPM.

Two motors of the same GCM rating will pull the same gear irrespective of the RPM at which they top out.
So when removing a King25 from a car and fitting a King30, I would not change the gearing INITIALLY.

However, the reason I change to a higher RPM is wholly down to speed down the main straight.
Yes, I know "races are won in the corners, not on the straight", BUT the main straight is by far the safest place to pass a car in an adjacent lane, especially in open wheel racing.

If you are thinking about a higher RPM motor because your car doesn't have any "punch" out of the corners , its likely that the motor you have is overgeared or is just not a good specimen. Test it! 

Some food for thought:
High RPM motors are really only useful on large tracks with long straights. 
Is the proxy track large? 
Maybe the proxies have no motor restrictions because the shorter tracks means that no advantage can be gained by a higher RPM motor. 

I've never raced proxies but it intrigues me. How to set a car up for a track you've never touched? 
For me , tuning is a process of continual evolution. Build, test, tune, test, tune, test, tune, test, tune.....
How to do that for a proxy?

You may find that the biggest challenge is not motors and gearing, it's TRACTION.
Getting the right tyres and preparing them for the track surface may well be your biggest challenge especially if it's not identical to your test track or local club track.

So back to your initial assumption: 
If you lower the gearing on a higher RPM motor , you put more effective torque to the wheels and this will exacerbate any shortfall in traction that your car has. Increasing torque beyond the capability of the tyres will make the car harder to handle and lengthen lap times. 

Alan
(Waiting for the rocks to start being thrown)