Question about offsets
 

I currently have 18X10 et52 rear wheels with 7mm spacers, which reduces my offset to 45. The tires that I have on the wheels are 265/35. Can I go with larger tires, like 275/35 or 285/30 without rubbing the struts or fenders?

Here are the tires:
Toyo Proxes T1R 285/30/18 have an overall tire width of 11.2

or

Toyo T1R 275/35/18 have an overall tire width of 10.7

I have a MY00 Boxster with the standard suspension, however I may upgrade to the M050 ROW set-up later.

It will be close - but should be OK (+/- 3MM)
We are at 42MM offset with 285/30/18 KUMHO V710 on 10" wheels and H&R lowering springs.
The tire diameter is important also - due to the spring location

Take care

Toyo tire diameters
 

For the 285/30 its: 24.7" (Fully inflated)

and the 275/35 its: 25.6" (Fully inflated)

Sounds close. Why are you set on running wider than 265/35? Unless you're on a suspension that can compensate, all you're going to get is more understeer....

Patrick