instead of worrying about a gas block with a rail on it, why dont you install a low pro gas block and put on one of the many different modular rail system handguards and put your sights on that. that will allow you to to mount any accessories you want and not have to put a stupid cheese grater (Quadrail) onto your firearm.
most of my AR's are configured with a handguard system like this, although i employ several different product lines.
here's a pic of my 223 with a Samson evolution showing the style of rail i'm discussing.. this isnt its final configuration (pre- accessory rails, etc) but itw as the first one i found to show you what i meant. i have a 2" rail at 6 o'clock for bipod attachment all the way up front, and another at 3 o'clock for mounting my LED light for night hunting.
anything you attach to your barrel (upper sight, bipod, grip, etc) via a railed gas block is going to mess with the barrels harmonics and accuracy.