The rpm hanging might not be the tb. It might be in your tune. If it was cable driven then i could believe that it was the tb it's self but being drive by wire and all electronics it makes me think it's buried in your tune.
GT-500 TB is 60mm so it's JUST a little smaller. If the electronics fit, I don't see it throwing a code either. My point is, if the 62mm BBK is plug and play and doesn't require a tune, the GT-500 shouldn't either. Doesn't the GT-500 require an adaptor? And, why are you getting rid of the BBK?
No i'm not saying it's bad. Try turning one way or the other with the bolt's loose and see if it moves. It might be turn just enough for the computer to think your pushing down on the throttle just a little.