First of all it happened so there is not much point in trying to figure out if was possible. Figure out how it happened. The P-51 has more weight and less drag than the La-7 so it should accelerate better in a dive. Furthermore the high weight/low drag should mean that it holds speed better than the La in level flight so after you both leveled the 51 probably did not decelerate as fast. Also the 51 may have unloaded during the dive which would have helped him considerably, particularly if you did not unload. If you have film, I believe you can get into his aircraft and watch the gages to see what is happening on his end. And as Ghosth points out, you don't know the 51's initial E state, he might have had considerably more smash than you to begin with, although Co-E I would expect him to close the distance a lot in the scenario you describe.
Hooligan