It seems to me that if the initial purchase was going to be for a transmission for the OBS then that would be the logical choice to go ahead put the entire package in there. That resolves the problem for the "acting" tranny and adds some beef. As for the XT6 those are pretty peppy as they are, though it would be fun it just wouldn't solve the tranny problem in the OBS.
Just my two cents...

<broke from spending last two cents>
-G