<S> sir!
The sim has no propwash effects affecting other planes in the vicinity. The issue with your plane being slower might be because the engine horsepower (hp) setup works in increments of 1000 ft in Warbirds. This means that your flightmodel has, say, 1200 hp between 18000-18999 ft, 1220 hp between 19000-19999 ft, and 1240 hp between 20000-20999 ft etc.
As such if your leader is flying at 21200 ft, and you are flying at 20800 ft, and the engine still has it's full throttle height (FTH) at a higher altitude than you both are flying, then his engines will be able to produce more horsepowers than your engines, even if you both are using the same throttle. Also his plane being at a higher altitude means his plane will experience less drag due to lower air density. The result is his plane holds a faster speed than yours.
If the FTH of the engine on the other hand is at, say, 20000 ft, the engine will then lose power rapidly above this altitude with every 1000 ft mark. If he is then flying at 21200 ft and you at 20800 ft, then your engine will produce more horsepowers than his engine at the same throttle.
* The speed of the B-24 Liberators increases with altitude up to 25000 ft (their FTH), above this altitude their speed drops rapidly.
* The B-17 Fortresses have their best speed between 25000-35000 ft, but above 35000 ft (FTH) it drops rapidly.
I can't say if this applies to your situation but it could be the explanation.
<S>
Robert