Is there any difference on group size or heating effects between a gas block with an iron sight on it or a low profile gas block types such as Larues, etc. if the barrel is free-floated.  Comments?

I have shot many barrels both with and without the gas-block and where some shot better without just as many shot better with the block.

Barrel harmonics are something that is fairly difficult to predict.

The difference is in HOW the gas block is attached.  If the gas block is a press fit, (ie, the inside diameter is so tight, they had to use a jig with a hydraulic jack to press it on the barrel....and it will never come loose), you will have good accuracy....all other factors being equal.  If the front sight / gas block is attached by tapered pins, with the pins hammered in place, you risk putting a small, but real, tight spot in your bore where the pins go through.  That isn't good for "match" accuracy, no matter who makes the barrel.  

You may think that big round barrel wont collapse a thousandth....how could anything that thick and heavy be affected by 2 pins driven through the front sight base??  Remember, barrel steel is very soft, and those pins are very hard, and if the barrel mechanic is "wanging" away a little to hard driving those pins in, you will get the tight spot.

If you have a pinned gas block, you just about have to fire lap the bore with lead bullets to get the bore uniform from front to back.  NECO used to sell a fire lapping kit that would do this... I presume others may now have similar products available.    I'd go for a pressed on gas block, IMHO.

i doubt you could ever tell the difference with any properly installed gas block

Quoted: The difference is in HOW the gas block is attached.  If the gas block is a press fit, (ie, the inside diameter is so tight, they had to use a jig with a hydraulic jack to press it on the barrel....and it will never come loose), you will have good .

No matter how you put a gas block on a barrel it will change the frequency at which it vibrates.  Sure, theoretically if you smash the taper-pins in hard enough you could add stress to that area of the barrel and consequently degrade accuracy.  I have hammered hundreds of match-grade ARs together, never lapped or fire-lapped any of them and have never seen any evidence that the block or pins caused any loss of accuracy.