There are some serious arguments against this. You will get more velocity going from 20" to 24", and therefore less bullet drop at various distances, but a longer barrel is not inherently more accurate than another, especially if you're shooting at fixed distances. Read Tubb's book "The competitive AR-15" if you get a chance. He talks about harmonics and inconsistencies that can disrupt a bullet in a longer barrel.
(Not to start an argument, Badshot.) :-D
It's all good, and if you do check the harmonics issue, it's with A. Lighter rounds (55 and 62grn.) B. Light/flutted FF barrels. The HB's really do suck up a bunch of that issue. Then again these are the same issues that we see in higher end "Sniper Rifles" as well. Might explain why there is a resurgence of the 20-22" barrels in that arena
Heavy barrel, FF and hell if you really are worried about harmonics (snicker) you can get the composite wrapped barrels that are all that and a bag of chips + a hefty price tag
On average
at the ranges we're really talking about, a 24" Hvy Barrel isn't going to suffer from enough issues to affect the
average LR shooter.
As for who makes a good 20" upper and barrel.. well, what are you willing to pay? WhiteOak Armament (I think that's them) supposedly makes the cats meow in LR uppers.
You can have a typical 20" upper accuratized by a good smith for far less. If you don't need the tacticool railed for-end, just get the carbon fiber tube with swivel mounts, have the smith verify the barrel mating to the reciever, the crown and what ever else might be needed for the uber accurate 20" AR.
Me, my not so little 20" RRA HB with it's heavy arsed MI FF rails does all I need it to and more considering I'm not nearly good enough to go beyond the ability of the rifle. I typically shoot well below 400M .. 600M on an odd trip out or two a year and my rifle does it's job rather well.
