I'm a big guy too with a size 16 boots and had the same concerns as you going to the narrow boards. I ended up putting the Cat pro-mtn narrow boards on anyway, but with screws in the outer rail and don't have any grip/slip issues. The only time I feel a little cramped is riding down the trail and I'm trying to slide my foot all the way up into the toe box area. Off trail I don't have any issues. I'll try to take a picture or two of what it looks like standing with my boots on.
I will say if I did it again I'd skip the Cat boards and spend the extra $100 on B&M. The inside plastic traction cleats I left on build some ice under certain conditions (the newer ones have a different shape to the cleats that might help this), I had to grind down a couple high spots of weld to get them to sit flat down like I wanted on the tunnel edge, and they aren't quite as beefy for a big guy jumping. I have a slight bow in them now. Not as bad as if it was a stock running board edge, but my previous early-build BDX tube boards were still straight after 3 seasons.