Great topic Phil, i will be watching this one with you! The only time i ever really checked the difference on my plane was a 25 fsr for slow combat on bladder pressure , lost about 800 rpm using a control line venturi over the r/c carb. I still run several fp's and a 46la with the r/c carb. It is expensive to change over, I ordered two venturi and needle valve assembly's to fit the os 40fp today and spent $53.00.

The R/C carb makes a remarkably good venturi setup.  The OS LA series carbs are pretty light and work well.  About the only negative is occasionally the barrel fit is too loose and it may leak air sporadically.  It's an excellent way to tune the motor to a particular plane and prop, since you can easily adjust the venturi size to find the correct opening for a specific combination.  Once you know what rpm you want, you can make/buy a venturi to hit that speed if you want.  The only real negative is the half-one ounce extra weight.  Especially on motors with large two needle carbs the throttle barrel alone can weigh an ounce.

A few comments on RC carbs.

I have been experimenting with Different Carbs on OS 25 engines.

I wondered why the plane bearing (FP) engine ran slower than a Ball Bearing (FSR) of the same size on the same prop and fuel.  The first thing I noticed was the difference in throat area.  The FP carb (2A) had an opening of .180 dia while the FSR carb (2B had an opening  of .216.  I have asked a friend to open the throttle hole in a couple of 2A throttles to the .216 diameter.  The only problem is that the barrel has been hardened and can only be ground to enlarge the hole.  An alternate to the 2B carb is get a 3A carb from an OS 40 FP that measures .205 dia.  That will get you a few hundred more revs.

Experimenting is no problem as all three carbs are interchangeable.
Clancy

I'd kind of guess today's carbs are also better and less restrictive than they used to be.  That might make a difference in whether or not it's worthwhile to replace the carb with a venturi on a given engine.

I have an OS Max .30 RC engine that is a mild easy handling engine that doesn't match my surviving Fox .19 baffle engine on a 9-4 prop for power, let alone a .25 Schnurle.  However, a few years ago, for SAM competition, I pulled the RC carb and turned a 0.200 venturi, and used an old thin Kap-Pak needle assembly.  The engine went from about 11000 max to over 14600 on 10% fuel.