thats cool man...only to glad to help
good to hear your goin 17's...
and in my opinion there isnt any gain when making the back rubber larger.
when you look at it, race cars like the v8 supercars (australian racing thing...dont know if you know about them over there lol) anyways they are curcuit racing cars and they have the same tyre width all round...
...so in my own personal preference i like running the same width tyres all round. large bak rubber is really only helpful when drag racing haha
but i do suggest whatever you do (because even 225's arent a thin tyre lol) look for a good wet whether tyre, i would worry bout finding a tyre that will dicipate water and snow quicker, rather than something better in dry conditions, so you might lose a bit of all round performance when you get the occasional dry day lol, but it will be much safer and more grippy in your more usual "damp" conditions
and i just got my supra a coupl of weeks ago...so if yours sits as high as mine (standard hieght)... then i highly doubt that there will b any rubbing at all even though the standard suspension is very soft hahaha...