Hey Tuke,
Welcome aboard. I've been running 19x8s with 235/40s and 19x9.5s with 275/35s (no spacers) for over a year with no issues. Prior to that, I had 18x8s and 18x10s on the car.
The Boxster suspension was beefed up by Porsche for the '98 model year to deal with the increased stress from the larger tire and wheel combinations. Apparently, there were some instances of the rear suspension tearing loose from the unibody on '97s with larger, grippier tires.
From everything I've read, however, I'm confident that you'd have to be pushing the car pretty hard to break the rear suspension. Like yours, my car is a cruiser. Though I wring it out from time to time, it's not a track car, and probably never will be.
The car will take 19x8.5s and 19x10.5s, but I'm not sure what spacers might be necessary to make them work. Contact Carlos at wheeldynamics.com. He's very knowledgeable and sells a pretty nice variety of wheel/tire combinations for Porsche. His prices aren't bad either.
Here's what I have going on...
