Yes, there are downsides when shimming...
No downsides of longer mast combined with the big wings.
But IMO loads of downsides of a shorter mast, but we can never agree on this one, I know.
It is a reminiscence from SUP that does not make sense in waves with a kite I would say, but some like the short ones for whatever reason.
When you shim the stabilizer, I have found the complete setup to feel bad when carving - you can shim the trim to be spot on to where you stand, but when you carve the dynamics seems to suffer if you shim the rear too much, both ways are wrong in fact, but at least if you shim to be able to stand further rear (like with the Moses, and another board I had) meaning rear stab not pulling down much or at all, the turning becomes stiff and odd.
In my experience - I dont know what others think?
Mast to fuselage shim will not alter turning abilities in this very respect at least, so a safer option if I am right you will alter the turning often to a worse state if too much.
But it has been a while since I experiemented with stab trim, as when I found out it didnt work well to change, I stopped and used the same trim always, just moved my feet instead, or mast position, to keep the "good feel" when carving
Small rear wing shim changes no problems though, for finetuning