245/40 x18 just clear on the front of my Cayman with stock wheels which are 8" and 57 offset. Your proposed tire will be about 2-3/4" wider or 1-3/8" per side thats about 34 mm so your offset will need to be about 23. I think using these tires in a square set-up will result in way too much front tire. If you do turbo or supercharge the car, the rears might be O.K. The largest rear tire I've seen on a road racing Cayman is a 305. I'd estimate that car had 320 hp
My cayman has 19x11 et40 rears with 295s and clears the strut by 1/8" (perfect). Not sure if the 986 rear geometry was the same, but I know it's fairly close. From the numbers Bill mentions above, you'd need alot less offset on the fronts (and flares on the rear if you want to run "square" wheels too) - then again you'll likely need flares on the front. You'll need to cut away front inner fenders for turning clearance, or limit rack travel - and the front scrub radius would be pretty nasty. However, I applaud your courage. Let me know if I can help and good luck. Will follow with interest.
Thanks for input! I was thinking 40 mm would be about right. No inner fenders left on my car. LOL!
You'd be suprized at how balanced the car is with square 9" slicks (they have 10.5 inch section width). I have big adjustable GT3 front bar up front and adjustable Tarret bar in rear which allow me to run square fairly easy. I know of a couple 986 Boxsters running 16x10 with 275 width tires under stock front fenders. Lots of advantages running shorter tires!
If I go biger I will likely cut fender and add flares or go with fiberglass GT3 RSR fenders and bumper, but budget may just keep me with my current 9" slick setup.
When I add up cost of custom wheels, bigger slicks, fiberglass etc - I'm not sure performance return is going to be that great. Tough to beat the $50 ea used 944 wheels and $200 cantilever slicks.
Go to CCwheel.com and use their conversion calculator. My 9.5" rears have a 7" backspace (44mm offset) and won't clear my PSS9s dividers/spacers on the front.... looks like a few mm shy of making it. I don't know what you have for your PSS9 springs - I suspect narrower springs than the standard 2.75" I.S. diameter I have.
As stated above, the conversion for 11" wheels, if you could clear with 7" backspacing would be 25mm offset. The outside clearance is another story.