when I wear a light shirt under it. I find just opening the buckles gets enough air in to cool down.
I'm like Paul, as one gets older and or, doesn't have the muscle tone one use to, the harness and or jacket is a way to stay in the game. From the point of view of one like yourself in great muscle tone and flexiblity, it can seem like a waste of time and sweat:-) I got one when LD decided to do his last match by WFTF rules and I couldn't use my harness. But, the jacket didn't replace it completely for back support and I can't shoot a long match comfortably and sucessfully without a jacket anymore. A jacket and a harness is even more steady on sitting shots too.
You've inspired me to work harder at getting more flexible and in tone, but I don't have high hopes for sucess:-)
I think you might consider one of these cheap and lighter ones to drag around with you at big matches for the offhand and kneeling shots... you probalbly WOULD, after a little practice, pick up another 25% in your positional shot points. Since your now coming after the 12fpe WFTF class, you might even tighten up your sitting game... that's a pretty tough class:-)
I have an extra one you can try when you get up here in a couple weeks.. if you like it, try it for the match.. it will all be in the shade anyway.