The physical differences between Rapide and Shadow (polished ports, slightly bigger carbs) don't make 10 bhp difference. What did make a difference was selective assembly. So it is not unrealistic to think of a continuum of twins, ranging all the way from piss-poor Rapides to super-duper Shadows, and in the middle a grey area where a good Rapide might actually be BETTER than a poor Shadow. Apparently the most critical things for performance were the camshafts, with good ones spot on and poor ones not. I have Gary Robinson "105" cams in my Shadow, and Gary uses a jig to position cams in wheels to ensure correct timing. I also have a set of NOS Lightning cams, untried, but which have been reset by Bill Jean (I think) so that the timing is correct. This makes me think that one thing that really OUGHT to be done "to avoid disappointment" is to check that valve timing is what it ought to be, and if it isn't, get it fixed.
Actually I am hoping to have my Rapide up and running with me on it by this Summer. But would like to feel the difference between a Rapide and a Black Shadow.