People who scale their sticks and trim up for turns are in effect moving the "more scaled" area of stick movement to a higher AOA. This makes it easier to stay at max AOA without going too far and losing lift. So trimming up makes tight turns easier. It won't make the perfect turn any tighter, but it could help make a good turn better.
Max elevator deflection is just that, MAX deflection.
All manually trimming will do is make your j/s throw shorter to hit max deflection.
So if you are pulling the j/s back an inch, or have trimmed up and are pulling you joystick back 1/2 and inch, either way you are reaching max deflection.
There are many times/reasons to trim your plane.
High speed in a ki84.
Nose up in an F4U @ stall speed.
Dropping flaps to land to offest the nose lift.
(there is more, just a few examples)
BUT to turn tighter in a stall fight isn't one of 'em.
(EDIT: after some thought, I wanted to add that I scale my j/s so most all the sliders are all the way at the top. There MIGHT be an argument to manually trim if you are using AH's default stick scale to help control a turn. Granted, you should prob. scale your j/s and eliminate that need, but there is an argument there.)
