Yep, you definitely cannot be competitive in Service rifle competition, without those heavy jackets, mine was heavy padded canvas. if you were comfortable wearing it, you were doing something wrong.
Offhand shooting was seen as a "poor test of a rifle" so said the guy who couldn't shoot offhand. If we are interested in testing a rifle, eliminate the human factor altogether.
I believe that once you have a rifle sighted in, all your practice should be offhand. If you can shoot offhand all the supported positions will take care of themselves.