Lowering the Box...

[Perfectlap]

By the way if you do plan to Autocross with your new suspsension PSS9 will bump you up a whole class with most racing clubs and you'll be competing against much more powerful cars. So from a competitive stand point its not really suited for Autocross unless you are doing it just to improve as a driver and not so concerned with placing. But around here, the guys in the more powerful cars are also the much more experienced drivers.
By the way Coilvovers are not overkill for a road car that never sees a track. Its a far better method of fine tunning ride height. When you lower that car on your new H&R or Eibach springs you are effectively stuck with that height like it or not. And if down the line you start to grow weary of the new lower height and miss the old more comfortable height, with colivovers you can return it to stock height in an hour. Reinstalling your old springs will take much longer and more $$$.

The best thing to do is go to a Porsche Club event either Autocross or Rallye (now that the season is back) and find a Boxster owner with a lowered suspension and similar wheels and tires. Ask for a ride along and you'll have a far better idea than just ordering from a catalog and crossing your fingers.