I have a couple of Kelly 3C's for cold weather work. I suspect that they'd be fine under the hot Sun, too. It's no fun putting a blisteringly hot mouthpiece to one's lips! I do find that they insulate, rather than conduct, so using them during moderate temperatures gives me the sensation of heating my lips, and I prefer the heat from my lips to be absorbed, rather than reflected.
They work like metal mouthpieces in every way, except for a slight lessening of complex overtones; the timbre is very slightly less full, but not enough to concern me, and not enough for most listeners to notice.
Using a plastic mouthpiece will change the balance of the horn, as it weighs significantly less than brass, and mute work is affected as well.
Kelly offers an inexpensive way to try some different mouthpiece sizes, and quality and durability are top notch, in my experience.