[Pat]

It sounds like your decision making process is solid. If I were in your shoes, I would definitely go for four tires. I understand cost is a significant consideration. That said, there are better tires out there for the price of the Sumitomos. They're not "bad" by any means, just not up to the level of a Boxster. That particular model is several years old, and newer tire technology has provided more bang for the buck.
I'm not sure what other tires are in the class of those Sumi's, so I cannot recommend an alternative. I can tell you Kumho Ecsta SPTs are an excellent tire for the dollar. That said, they are a bit more than the Sumi's. If I were in your shoes, I'd give the new Yoko S.drive a try. I had a few sets of the ES100s, which I loved. The S.drive is Yoko's replacement for that tire, and just hit the US market a month ago. A little more than the SPTs, but still a solid value for the dollar, from the reviews I've read and the history of the ES100.
I know that's probably more than what you'd like to spend, but if you have the means, that's probably the smart, value-oriented way to breathe some new life and performance into the car.