I had to play with those settings for a bit, but after smashing my face against my 4K TV and doing everything step by step, I got it filling the entire screen using the SNES Video Test Program. Honestly, I find it pretty weird that some values have to go on the minus side of things, and my Mario Kart ROM loads perfectly with SNES9x and Bsnes, but not with Higan. Maybe this core isn't ready for prime time?
This are my settings for 4K res, if anybody wants them.