There seems to be a lot of confusion, let's boil it down.
In a normal turn increasing trim will not improve turn performance. Period, dot, end of story.
In a high-speed dive with certain aircraft, such as the 109 and 38, flight loads or compression effects can require much more force to move the elevator full throw than the pilot has available. That's the only condition where trim can help but that condition shouldn't be grouped with "turns" per sei, it's a dive recovery and, technically, it's beyond the normal operating envelope of the aircraft concerned. Someone who's diving in on a bandit so fast that they're needing trim to "get the shot" should probably be more concerned about how they got in that situation and how they're going to get out of it rather than shooting someone.
Technically, is a dive recovery a turn? Sure, technically it is, but since it only occurs in that particular condition, is outside of the normal operating envelope, and has unique attributes (effect of airloads/trans-sonic air flow) it's not really the same as talking about how many degrees per second or the turn radius you can read off of an EM diagram, i.e., traditional "turn performance" numbers. The big point is that it isn't a factor in a turn fight so it's confusing and misleading for folks to claim trim improves "turn performance" unless they're speaking strickly of dive recovery in which case they should call it what it is, a dive recovery. This confusion is why so many people claim trim is some "secret" that will let them match a Zeke's turn in an F6F. It's not going to "let you turn faster and get the kill rather than being killed."
