I ended up putting on a quad rail just because of the limited selection of after-market backup sights.
I much prefer the standard handguard, but the MI flip-up felt really loose when 'locked' up - the Magpul MBUS Gen 2 is worth the slightly inferior foregrip.
I emailed magpul, and they said the MBUS Pro /might/ be available for the 556/522 gas block fitting - if so, I'll almost certainly take off the quad-rail.