I recently did reversal speed tests. this i did by flying at level speed to 400 mph then executing a slight nose down flat(ish) turn pulling or at least trying to pull a steady G ( i think i used 2.5 or 3 G) almost Every aircraft performed similarly. I found no difference or at least very little between high wing loaded aircraft or low wingloaded. The point was that i kept within the high speed stall limits and as a result lost little speed in ANY aircraft.
(add here the old turn speed/turn rate debate

)
Had i jammed the stick back in a jerking movement and 'mushed' the aircraft, (i.e. turn so tight the airflow over the wings is disturbed thus losing lift and producing more drag) then the results would be much more AIRCRAFT MODEL specific. Then the acceleration after the stall would be of greater importance rather than the ability to hold speed through a non-stalled reversal.
One thing you should realise is if an aircraft passes your front quarter at a level atittude he could be travelling at 200 or 600 and you will find it hard to judge it perfectly. Lets say for arguements sake they are at 500mph and you are at 300mph.If they execute high G flat reversal/slightly nose down to help keep speed they may lose 50 mph(if they are good it can be less!) but they will end up at your 6 oclock with a 150mph speed advantage.
If your aircraft was to perform the same reversal at 300mph you too would lose some speed but you would STILL be up to 200 mph slower than the other aircraft.
To one it appears the other has reversed with no loss of energy, to the other it is completely different. You have to watch them as they come out of the reversal IN ORDER to guess their E state. Its incredibly difficult to judge it BEFORE he exits (unless of course yo KNOW their speed which i assume you dont in the situation mentioned )
Having said that if you were in a 190a5 at maximum speed, then dive with wep and then extend and you are at absolute maximum speed (i.e. your aircraft is shaking) then its HIGHLY UNLIKELY that any aircraft could pass you in the opersite direction and reverse and then gain on you WITHOUT having an initial high speed dive to put him at HIS maximum speed.
The 190 is no slouch BUT it does lose speed and energy quickly. Performing rolls etc to avoid flack DOES slow you down quickly yet it 'FEELS' as though you are losing little. Are you sure you knew all the paremeters of this fight? are you sure the p38 wasnt just out of a high speed dive? have you tested and managed to remember how the many different types of aircraft behave in the manouver you described(and have you done it recently)? are you sure you didnt bleed off more E than you thought? if you are sure then you could have something but if you arent sure I'd put it down to experience and be aware things can fool you into thinking you have it all correct when you dont.A film is the only real way to prove things one way or the other.