This is good to hear that BB92 with GXP has no issues! I live in a flat land with some hills around so I need gear ratios from 4.0 down to 1.0 as the bike will be like a cyclocross but a mtb which is stronger frame..
With 1x11 I ran into issues with frame clearance around the chainring, you are correct, so it looks like I'll use a 2x10 with 42T biggest ring which should fit. There is a thread about this from me with some frame pics.