IF I recall correctly, yes I believe -1.5 degrees of camber is possible in the front with the stock suspension, but it's been years since I had a stock suspension - first lowering springs and then coilovers, so I could be remembering wrong. I was thinking of checking the Boxster Spec board to see what those guys run, but they're all using PSS9 coilovers....
Having so much negative camber in the rear is more a function of having the car lowered so much rather than a desire to actually run that much negative camber! When you drop the Box to an extreme level you're going to have high negative camber in the rear if you're also going to adjust the toe to slightly in. The two (camber and toe) are very much tied together. The only way to dial out some of that negative camber is with adjustable toe arms so that you can get BOTH good toe AND less negative camber. I don't have adjustable toe arms and don't really care to get them as the high negative camber in the rear does not concern me (as in, I don't care about the cost of higher tire wear).
The general consensus on how much negative camber you want is as much as possible. This is for track and autocross though. You have to be willing to throw away some money on tires to run this way. If I was going to seriously track my car I would slap the Schnell camber plates back on the front and run around -2.5 degrees of camber. Right now though I'm just autocrossing and not too serious about it, so I've got a more mild setup in the front. I have heard many times that slight toe out in the front will help with autocross, but again I've chosen to set my car up more for the road and secondarily for autocross, so I've made some compromises.
Hope that helps.
If you want to autocross and track your car - here's what you tell the alignment shop:
Max negative camber front and rear
Zero toe in the front
Slight toe in at the rear
IMHO....
Kirk