I agree that a leadership change alone would not be enough to prevent a comfortable Tory victory in 1983. You'd probably need to at least prevent the Falklands from happening if you wanted to change the result. Even then, Labour infighting is obviously still going to be an issue. If a right winger were in charge, Benn might even and try and challenge for the leadership, rather than deputy, and its possible he could even win. At the same time, having someone more moderate than Foot would probably persuade the Gang of Three to remain, but Jenkins had decided from fairly early on that he would found a new centre party, and we'd probably still see that happen in that situation (although it maybe a different kind of party to the SDP), taking a few Jenkinsite MPs with him. Ultimately, although Labour would still do better than OTL, the best they could hope for would probably be only very minor gains of about a dozen over 1979, or more likely a net loss of a similar number of seats.
So whoever beats Foot is likely to be moved along shortly after the election. And of course, with the right having just led Labour to two landslide defeats, any contest to replace them favours a candidate from the left. Plus a better 1983 result probably means that Benn has kept his seat and can now run for the leadership. It could be that no Foot would just mean an even more extreme leadership in 1983-87, with an even more extreme version of the suicide note, and another SDP style breakaway.
More likely, though, is that Kinnock or someone else from the soft left faction is able to beat Benn. The different circumstances from OTL might mean some kind of shift to the left, but maybe not as far as 1983- unilateral disarmament, EEC withdrawal, and renationalisation of privatised utilities would all likely become party policy. That would probably mean that 1987 is a defeat on the same lines as 1983, either involving very small gains or minor losses. The margins involved make it difficult to say whether they could still win in 1992, or even say for sure who the leaders of the major parties would be.