First off, if you are going to get them bored do a bit more research. The front bore is 70.XXmm, I can't remember exactly but if you have them done to 70.00mm they won't fit.

I don't think adding a spacer to the rear really qualifies as keeping the setup staggered. Also I'm sure you realize that you are going to change the handling of the car (not in a good way). Adding more grip to the front (slightly wider tire) and taking away grip in the rear (skinnier tire) is going to exagerate the tail happy characteristics of the car (more oversteer).

Not trying to bash, but you are taking a great handling sports car and ruining the balance of it to save a few bucks on tires. I guess if you just put around town this might be ok, but if you do any sort of aggressive driving this will be a major downgrade IMO.

 

70.3 for the front or use a spacer to clear the lip on the hub that your wheel bores won't clear. obviously, that's gonna depend on the wheel widths and offsets and whether or not you feel safe with 3 or 5mm worth of less lug thread