I have to agree with CJ's last paragraph since I have some recent first hand experience with this. I did build another AR with a free floated hand guard because I am using it at 300yrds+. However, my Sport II is stock, and IMHO, unless you are shooting past 300yrds, you will not notice much benefit in accuracy from a free floated hand guard until the barrel gets heated up. It will take several rounds for the barrel to heat up enough to make a difference. Of course, the type of ammo and how fast you fire those rounds will make a difference, but for the average shooter, I'd say you would need to fire at least 20rds in quick succession before the barrel would be hot enough to notice any difference in accuracy at less than 300yrds. If you are the typical target shooter who will shoot a group or two and then let the rifle cool while you check results or make adjustments, then you are never going to notice any difference.
If you are the type who is going to empty 30rd mags as quick as you can just for the sake of doing so, then you will want a heat shield and/or a free floated guard to help get rid of the heat, but accuracy is going to be out the window no matter which hand guard you have.