Since the debate's still going on, here's my reason for giving up on the different forms.
It's a fair assumption that most of any audience in this country are familiar with the word "bravo". However if someone next to a casual ballet or opera goer starts using "brava" or "bravi", that might lead them to feel that they don't belong at (for example) ROH; that there's some special code of behaviour they don't measure up to. This reinforces those stereotypes which many of us on this forum and all the ballet companies fight so hard against.
My understanding of good manners has always been that, in essence, they are about putting others at their ease (unless their behaviour merits otherwise of course!). I can't claim always to have lived up to that standard, but I'd rather Punter A thought poorly of me for not knowing my Italian endings (call me arrogant, but it's enough that I know I know them) than Punter B wondered if they really fitted in at the opera because of me. On all counts, Punter B is more worthy of my consideration.
(Side note: I'm not really a shouter either - if I'm really pleased with something I'll go so far as to say something like "oh, bravo".)