The SUs are a bit easier to tune as you simply spin the adjuster up or down to change the jetting but bike carbs (or any others for that matter) are easy enough to tune. To get that last 315 horsepower does take a bit of knowledge but you can get them 95% with not a lot of effort. Either fitting an air fuel ratio gauge or taking it to a dyno place to tune it will get it done. There is a ton of how to jetting info on the web.

If you have a GX manifold and the SUs already I'd stick with that. You can fit a set of twin bike carbs; loads of British car guys are using a set of twin Mikuni carbs to good effect.

Tom