On comms or not? If so, as the guy. They'll usually say go ahead. Was the P40 a serious threat or not? If so, take it out. Even if you go ahead without mentioning anything, you were in the better position. The best example of this is when some twit is 3-400 m off a bomber's 6, and I swoop dwon in a classic slash attack, and the dufus screams "stop cutting in!!" WHILE he gets his a$$ shot to hell by the bomber! I'm attacking correctly, and you're not, stupid!
Anyway, if you "cut in" in this manner, Highlander, you're taking advantage of a superior position. No good pilot would (should) bitch at you about it.