It's the Ultima I'm not really excited about. I have a chain secondary drive so ratios are pretty easy to change (of course parts aren't always within reach here). Most OD6's go from 1:1 in 5th to .86:1 in 6th, so there's a little more drag in 6th. No big deal really & on some bikes ya can't even feel it. With the Ultima I got, it seems there's tighter gear meshing in 6th; hence, the nasty whining there & more drag. Also (not really a big deal) neutral's harder to get, the extra neutral light in 4rth & 4rth has a bit of a whine.
From what you're saying, sounds like an OD6 is what you're after. But if it's a long term keeper, I'd research for some better quality IMHO. With the HP & TQ #'s I seen you post, you could easily get away with a couple more teeth on your front sprocket. That would shave a few hundred rpm's from up top, cheap, easy days job & you could use your same belt.
Be advised, that 17" rear wheel you were talking about, depending on the profile tire, that will change gearing in its self to some extent.