I would agree with Tac in the case of YAK vs. P38.
At the end of january i flew YAK for a week or so, and it was no big think to kill low P38 from a co-E situation, especially of the P38 went into loops.
The key, seems that the YAK must constantly fly lag pursuit, up to the point were both planes are over the top, than the YAK can use its incredible accel, to get a speed advantage over the P38 and trade that next turn for angles.
If the P38 notices that its on the loosing end, and drops out of the loop, starting a scissors or so, again, lag pursuit will do it. Also u can chop throttle, hang the YAK behind the P38s scissors. If the 38 than notices that there will be no overshoot and thinks he can accel away, the YAK simply rams in full throttle and u will catch up with the P38 fast.
Cause i normally fly 190, i can say that a YAK suits much better to knifefight with a P38. If you use some good old E reserving moves with the YAK, the P38 will not get home.