There is nothign more I have to add to this question.

My long underworks one (997) does help me stand up straight, kinda like lumbar support.

I use the tri-top (style 983), and it doesn't help with my posture. I'm thinking of getting a posture brace after top surgery.

My 997 helps my posture a bit and really helps my back in general. Pre-binder I experienced a great deal of back pain due to an injury I got when I was 17 - my girlfriend is a doll and used to give me weekly back rubs to ease the tension. Since buying my 997 I barely experience back pain - so no more back rubs.

I've always had good posture though, but I definitely feel that the binder helps to reinforce my posture. So if you slouch it should help.

I slouch when I'm not binding to hide my chest. So yes, my posture improves when there is less to hide.