What do you think of 245/45-17 Victoracers? On 8" rims they ought to have a section width of 9.7", which is the same as S-03 tires in 235/40 on an 8.5" rim. Much as I would like to see 255's, it seems unlikely without a bit of body work. I'm not sure I want to roll my fenders just for the small amount of extra patch over 245s. Until I get the car I won't know what will fit anyway.
If I do go non-stock width rim, I'm thinking 8.5" since that fits a lot of big tires (255/45 Hoosiers, 255/40 Victoracers) as well as the more sane 245/235/225s. 9" might be too wide for a lot of 225 tires, but maybe you won't care about ever running something that small.
According to my G-Cube, my stock non-R Miata with 195/60-14 Falken Azenis tires typically exceeds 1g at transition. Sweepers look more like 0.8g or so sustained. Good R-compound tires would help immensely, and the increased patch from big tires, and decreased body roll (I hope!) of the STi would help as well.