a 275/50/17 is 27.83inches tall....255/55/17 is 28.04 inches tall....and lastly a 275/55/17 is 28.91inches tall. I'm sure you know already know this but a 275 is the widest you'd want to go on a 9.5 I think a 55 series would be way too much sidewall and besides not being that aestically pleasing, it seems to me it would defeat the purpose of having 17s so you can run a bit shorter, stiffer sidewall. I have seen 275/50/17s on the back of a few newer mustangs and it doesn't look bad, and since its just a tad under 28inches its prob. your best bet

I know for a fact that Falken and Nitto both make 275/50/17s. If you really want to go the 55 series I would suggest going to
www.tirerack.com as they have a bunch of 255/55/17s and 275/55/17s, but I wouldn't be surprised if a few people joined the vote against the 55 series. Hope that helps!

and please remember its just my opinion