Kneeling like anything else can be a stable positon if done right. This is where a good coach comes in. If you can't shoot a rapid fire string in kneeling, or any other position there is something wrong with the position.
A natural point of aim should allow you to rock back into position for the next shot regardless of position.
I can attest to the fact that shooting with a heavy coat, and layers of sweat shirts raise the body temp. Shoot a lot of matches in 100 degree temps with 80+ humity, and I'm sure my body temp went up, but it didn't help my shooting.
We use to soak down the sweat****s and rap the coat in ice before shooting.
Yes I do believe the heavier the coat, the better your offhand scores will be, but as far setting (or kneeling) and prone, I don't think they helped that much, but they don't hurt anything either. I'd recommend the heavier coat to anyone getting started.
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071