This will be interesting. I have 18x8.5 all around, running 245x40. I've been thinking of going to 255x40 next time. I have a ET25 wheel. Staggered fitments are out of my knowledge base, but if other MB products are any indicator, I would think 275x35 on the 9.5 wheel, maybe even 285. I think my rear 19" S55 wheels are 9.5", and they came stock wearing 275x35. I went to 285x35, small change, more cushion for the pushin', and have been pleased not to really notice. It's not quite as harsh is all, which is welcome. There's a "GSXR" in the S55 forum on MBWorld who had worked through that one, so I followed his lead, as the question was on my mind anyway.
So 245 front, 275 rear should be safe depending upon your offset. But you can probably go 255 front, 285 rear. I think this is squarely a GSXR question.
Cheers,
maw