Adding spacers with factory 18s

[geetee]

so all the factory cars that came with wheel spacers are going to ruin their suspension?

many porsche models came from factory with OEM spacers with no issues running for many many miles. Factory spacers ranged from 5mm to 17mm's.
Make sure they are hub and wheel centric and you're good to go

[boxxster]

A reasonably sized wheel spacer is not going to ruin your suspension. There is no difference between adding wheel spacers and putting a wheel on that is the same width and diameter but a slightly lower offset. Either way you're pushing the wheel outwards and it will be putting more leverage on the suspension. If you go with a spacer no bigger than 10-12mm, I can't see there being any problems. You'll still be far better off than the people who put 10 and 11" wide wheels on these cars.

[Perfectlap]

Heavy wheels will ruin your suspension. Heavy wheels with the wrong spacers will punish your wheel bearings AND suspension. If you track or autocross then you definitely need to speak to an experienced professional about your set up. Correctly spaced wheels will not damage your suspension, nobody is claiming that. But all too often I see people buy a second hand Porsche and slap on heavy aftermarket wheels and spacers. The car is now slower off the line and your giving some very expensive parts a premature death.

Are these 18" Turbo twists with rear Boxster fit or are they possibly the solid spoke twists from a 996 Carrera?
If the latter, those are not the lightest wheels, they're nearly 30 pounds.