Yet another tire size question...
 

I'm sure this has been covered, and I've seen at least one page suggesting they'll fit but I'm skeptical. Will 275/40ZR17 rear tires and 255/40/ZR17 fronts fit on stock rims?

Here's the site I found that said, yes they'll fit but that seems like a huge bump.
Boxster Tires and Wheels

I have run 255 x 18's in front with a Dunlop Z1, and 285 x 18's in rear with a Hoosier A6 (obviously not at the same time). However, on stock wheel width's, I would expect a 255 and a 275 would be beyond the tire manufacturer's recommended width, so the tire profile would not be ideal.

Take a look at the recommended wheel widths in the specifications tables on tirerack.com. In my experience you don't want to go much beyond the tire manufacturer's recommended wheel width.

They are track tires so I think the the stiffer sidewall would help a bit. They're well beyond tire tack recommendations

Don't forget these taller than stock tires will reduce torque multiplication & raise ride height.

I think stiffer sidewalls are actually a negative for fitting narrower rims then intended, as the stiffer side wall will cause more deformation of the tread when it is rolled onto a smaller rim.

I have made measurements for this with NT-01's, which show that if you go over the recommended wheel width the overall tire width will be reduced by approximately half the difference in wheel width. That is if you take the same tire measured on a 10" wheel and mount it on a 9" wheel. you will lose 1/2" of the rated tire width.

A few months ago, GRM magazine did a back to back tire test with BFG Rivals and ZII's, where they changed the wheel width for several runs, and it made a large difference in tire performance with no other changes.

Bottom line seems to be that if you want to take advantage of some really wide tires, you need some really wide wheels.