I believe there is a right way and a wrong way for my modifications (emphases on mine ). You have done a lot of reading to find that post about tolerances between uppers and lowers and and you are damn right, I do like tight tolerances for my builds. When I build a gun I want it to be the best gun I can build. Paying close attention to details says there is no shame in my game. If you don't mind cutting the sight post down then more power to ya. I am just saying I would buy a 20 dollar gas block with a rail on top, dimple the rear set screw and do it professionally. Knowing what you are doing with build and repairing AR 15's has nothing to do with a hacksaw. Now if the the owner modifying the front sight post cut it down and used a dremel tool or even a file and rounded of the edges and hit it with a little black paint then all would be good. For me cutting down the post and wrapping it in black tap is not the answer. A 9 dollar riser would have made all that nonsense unnecessary to begin with. Changing the gas block or using a 1/2 to 1 inch riser is knowing about AR's. There is always a part that will do the job right and knowing those parts and using those parts is the difference between knowing the AR platform and inpatients or all out ignorance. I am sorry for going there but don't attack me for wanting to do something the right way. It's your gun and you can do anything you want to with it I don't care I just don't see that as being the right modification for the end result.The only reason I would cut one down would be because I was putting a longer rail on it.
I also prefer an FSB but to say shaving one down is a "no no" is crap.
An AR 'builder' that can't see a reason to ever cut down a factory FSB doesn't know what he's talking about.
But as long as he can't see light between his uppers & lowers he'll be an expert in his own mind.
Edit to add : and as long as he has plenty of buffer spring grease.