Those who have flown the P-51 will know how painful it can be, with the 50 cals nerfed yet again and the sideclimbing to attain the same altitude that can be reached and surpassed by everything it will face. But it's not really the fault of the P-51, it was built specifically for bomber escort. It was never meant to climb as fast as say German interceptors, which did need to climb to reach bomber formations. (This applies to other American aircraft too, but thats a discussion for later) You have other aircraft like the P-38s and P-47s which are given air spawn, and they are the difference for allied team because they dont need to spend half the battle getting to alt while their team loses.
To make the P-51 more viable/actually enjoyable to use, I think giving an interceptor spawn like what they did to the P-61 would really help. And I know this one is a stretch, but allow the P-51s to spawn with the heavy bombers for escort. This could be furthered by adding a new reward system where getting a kill withing a kilometer of a friendly Heavy bomber will give extra SL and RP, but I think this would mainly benefit American and British Heavy Bombers as other nations have other ways of fending for themselves (German Bombers after the 111s and FW 200 have the speed advantage, Russian heavies are Russian, Japanese bombers are bristling with cannons) and with this players will have an intensive to protect their heavies, allowing the Bombers to have their adamantium armor taken away.
Now you could say that people might try and abuse this, but this is why I dont think that will happen.
1. Starting at 4km up means you already have plenty of alt, and climbing further is just wasting your time.
2. Extra reward is very enticing for those that love to get as much RP as possible, and the 1km range requirement means they won't be straying far as long as they want the reward.
3. Going ahead of the bomber formation and attacking German fighters is another one of the things P-51s partook in, they would attempt to break up German formations to disrupt their attempts to intercept the bombers. And besides, this 4km air spawn will only be given if the team has B-17s (Possibly make it so it only allows the spawn if its 4 B-17s, and in this case it would be even more reasonable to give the spawn because the Americans are already down 4 fighters in this scenario)
Let me know what you think about this, and how this could change the Bomber meta.