how much (if any) did it affect your accuracy? Thanks.

Not much different for the average shooter. If you are not able to 'one hole' it at say 75 yards with a 4x optic and good ammo, a free floated barrel will not make up the difference.

Now, if you are wanting to pull a 12" group at 500 yards......then it might help a bit. Otherwise, not really necessary in the least bit.

I think it makes more of a difference if you intend to use it for accessories such as a bipod or a foregrip..

chances are anything you attach to a non free floated rail system will change the point of impact when pressure it put on whatever is attached.

I put a cheap UTG free float rail on my MP15 Sport, its extremely sturdy and works well.

I agree!

Money would be much more wisely spent on a really good trigger system if improving target accuracy is the goal.

Yes. By far a better investment. A determined shooter will learn any trigger they have to work with, and overcome most of the shortcomings. But you really can not compare a 7.5lb AR trigger that breaks a bit slow to say the factory 3.5lb trigger on my Howa 1500 .243 It's got zero creep, and zero overpull. Just press back, and Bang. Perfect.
A good shooting cabine has a different trigger on it, because well- it has a different purpose than a pure paper puncher or a serious hunting rifle.

Great replies, Thanks!