Eomak,
I have a 2001 986 lowered with HR springs and my rear wheels are 18 x 10 et 65 and I run 21mm spacers in the rear.
I tried 15mm and 18mm first and I had issues with the strut hitting with those, the 21 work great for me. You will also need longer wheel bolts.
The wheel spacers can be both used aesthetically and necessary depending on the wheel you run. Aesthetically, they will push your wheels further out from the car and I think that looks better as it gives the appearance of filling up the wheel wells better. By necessary, my aftermarket Carrera lightweights have a different offset than stock Boxster wheels, so in order to get them pushed out far enough that the wheel doesn't hit a strut or anything else, I had to use spacers.
The long term effects on the suspension for me are yet to be determined.
Mike
