Grumps - I already use short-shank vintage Bb's (a 'slant' Link 6*, and a Mojo Meyer .069) on the C, which both respond far better over the whole range than the original 'C' - which I'm loath to mod as they are a bit scarce. Although I've used up to 8* on tenor, I don't want to go much more open than those two on C-Sop (They are fine for my more modern Bb sop). I'm beginning to think it's partly a psychological thing, possibly because the two in-line palm keys feel a tad awkward.
And the original C mouthpiece (still a sweet sound, just a bit inflexible) gives me a handy datum against any Bb's I try. Ed Pillinger, in the UK, does a good custom C-Sop mouthpiece, I may treat myself to one before long.
I mustn't digress, so I'll get back on the 'nino topic by saying that maybe the C-Sop is a useful step between Bb-Sop and Eb-Nino ? However, I think, because I've already gone closer to a 'nino with the C, (funds allowing) I'd probably want to go for broke and get a soprillo instead ! Looks like fun, but I do worry about the dog's reaction tho'...