Tire Width vs hp
 

Can anyone tell me: if I bite the bullet and upgrade from my 3.2 stock S engine to a 996 or Raby build engine with another 75 hp, am I going to need to go wider than 9"rears to get the power to the ground? As it is, I have PSM with virtually no spin, which I think would continue but perhaps with underutilization of the extra hp. Thanks for any informed insight.

There is no hard and fast rule but most guys who run 300rwhp Boxsters at the top of the podium are also running 245/275 R-Comp or Slicks. They also see a big advantage in a motorsport LSD. A 275 tire will fit on a 9" rim. For street use it is a non-issue.

There's no reason you would HAVE to go to a wider tire.