I would be careful about staggering the wheels, due to the difference in rolling resistance. From a braking perspective, the S2000 is engineered by Honda to have a staggered setup. I have considered the same thing, since my MR2 had staggered wheels on it that will fit the Z and hopefully sit flush too. But, I have to pick and chose my tires if I decide to do it, because of the difference between the F/R braking force.
Oh, and I don't think the Z will be able to get a 9" rear tire under it. On the MR2, a 255 would be a very tight squeeze, and it was coming from a 205/55-14.
Try the 1010tires.com sizing calculator. Very helpful