the unions are pretty narrow and ive read some things about the NOW bindings being a nice snug fit(although i havent tried any NOWs personally). what issues are you running into that you feel you need something narrower? ive had a good 5 or 6 pairs of burton bindings over the years and never had issues with my boot sliding around in the binding or anything.
I wanted to move up to a size "large" binding without having to much slop on the sides. The Burton chassis leaves too much room on the sides of my Nike Kaijus if I go from my normal medium to large in Burton.
But I picked up a pair of "large" Flux RL which fit my Nike Kaijus very nicely. Looking forward to taking them for a spin tomorrow. I'm using some Elan boards that have an unusual insert pattern called "boomer stance" and it has made getting lined up on my sweet spot a little difficult. But these Flux bindings did the trick. I'm surprised how well made they are. Can't believe I've never tried a pair.