[Spook]

Jon, i'm running 275/40r17 on the rear and 255/40r17 on the front and car is lowered 1" front, 1 1/2 rear with no rub. Just make sure the correct offset is used.
I beleive the C2 use a different (newer) offset. Like my 1986 951 you will need the correct wheel adaptors for proper fitting otherwise the wheel will be pushed into the fenderwell. The 86 year as you may know have a different suspension from any porsche 87 and up and use wheels of different offset.
I have boxter wheels which have the same offset as the C2 and i as you have started with 7's front and 9's rear. I now run 8 1/2 on the front.
1st you must mount the rim to the car and measure the available space between the fender and the inner fender. You can use the "tire rack web site" to get the tread width in their spec's page so you have an idea of available space. Remember when trying to fit the biggest tire width in an available space you must give proper spacing (preferably an inch) to compensate when the car is going around a hard corner or carrying additional weight other than the driver.
I am using the widest tread width under my 951 without rub. if you want to play it safe use a size smaller.
If my post has an interest to you i will try and dig up the wheel adaptor size i purchased thru "Performance Product's", HR has the same adaptors.
G?L