7.5 x 17" is the size of the oem wheels as stamped on the inside .
275/55/17 although it will give you +/- the same rolling diametre it will be bulging outside your wheel and not advisable!
Regarding the difference in rolling diametre,this is a negligible .98% and with the 275 you will be riding only 2.5mm lower than stock.Us guys the oem 20"s are riding some 9mm lower than stock,so you are definitely ok.