Contact Greg Fordahl at Fordahl MotorSports. I spoke to him at Nationals in 2010 and he can work on custom valved external adjustable Cayman shocks that work with the stock springs and struts. Making them work with the stock struts, springs and strut tops is the difficult part, and Greg already knows how. If you want to do the same with a 996 Carrera for A-stock, Greg is the guy.
The car needs way stiffer shocks, given the soft springs.
The car with PASM has stiffer springs, so that's the ideal car. The Sport Chrono package when set to Sport has a better response from the gas pedal (the stock cayman gas pedal is a little laggy). The Cayman S Sport Edition (303 Hp) can only use 19" in A-Stock per the rules, so somehow that car has to be converted to non-Sport Edition to be able to run 18" wheels. Find a standard Cayman S with PASM and Sport Chrono.
Almost impossible to find a 2008 Cayman S with PCCB and the optional carbon seats. To retrofit the seats is $6,500 plus labor, and the brakes another $15,000 plus labor (new uprights needed). The ideal car is 100 lbs lighter than my car at Nationals, and we weighed at 2,960 lbs running on the fuel fumes.
I have run the car on 18 and 19, the car on 18 is a lot faster (nor barely, but a lot). The 19" 295 is a 26.1" tall tire, the 285x18 is 24.9" and wider than the 295, much better gearing in 2nd gear for autoX, the car stays over 4,000 rpm most of the time (where the cam plays nicely up to redline).
Max-out front camber, and run the minimum possible rear camber, 0 toe rear, a little toe-out front. Don't forget to install the 997 GT3 front sway bar, in the softest or one from softest setting ( I ran it in the middle at Nationals, and every co-driver complained about massive understeer).
I'm getting a very long life out of my 285/30R18 A6 in the Cayman with just 1 degree of negative camber (maxed out front, minimum rear). On my practice wheels (Chinese heavy knock-off Porsche wheels), I have a set of A6 that has 56 autoX laps, 25 laps around the Miami Speedway (HomeStead), 200+ street miles, and they are still good, I have never flipped them. I will run them next month at Homestead until I cord them.
I have followed the Cayman prices in the last 2 years, and they hit rock bottom a year ago, now they look a little higher. It's a nice car that needs lots of power.
Reading your set-up with interest but some questions please... Does the smaller OD on the 18" wheels/tires create problems with ABS on your Cayman S? Also, are you running the 285x18 on all four corners? If so, how well does that work on those 8.5" front wheels? Hoosier specs call for 10-11" wide wheel.