I have a '90's era Colt H-bar, an -A2 style with fixed carry handle. For many years I considered swapping the upper, floating the barrel, and other stuffs... but decided in the end that it shot well enough to not mess with it (this outside the value of a gen-u-wine Colt for resale, or it's 'pre-ban' value in some states.) It's almost as inexpensive to just order or build a complete upper and swap them between your lower receiver (assuming it's not something weird like my 'big pin' Colt lower.)
As far as barrel heat... the barrel profile has a lot to do with it, too. Sustained fire through the heavy barrel of my Colt, with it's aluminum insert handguards, is never a problem. I had a RRA lightweight upper built, with a light 16" barrel... and that thing gets HOT. Fast. We're talking 100 rounds and you have to wear a glove to hold the thing. I have a Magpul forearm, and I don't think it deflects heat like the Colt USGI-style ones do, and with a free-float forearm you have no heat deflectors, period.