The 3/4" didn't make the X-brace lower than anything else under there, but the rounded rear part of the brace did drag over a couple of speed bumps.
In answer to your questions;
IMHO - it's a little loud (remember I'm older) and I think Convertibles attract enough attention without the extra noise.
Yes - with minor mod.s
Yes & No - Flowmaster has a crossover type that's quieter, but I'm not sure you could call it an "upgrade" as it doesn't flow as good.
Yes & Yes - the pipes bolted okay to the collectors, but we had to rebend the pipes to line up and if I had it to do over again I would have cut a little out of pipes to move the assembly forward for more axle clearance. There are many variations in collector location and we got this system a little to long during fab.