Built for occasional autobahn jaunts, it doesn't surprise me the Boxster can maintain those speeds pretty well. I did the same early on, one full speed trial just to see and never really had interest to repeat since then. I got up to 152 at 3am in the morning on a deserted 4-lane freeway. The engine was slowly working it's way at the top of 5th into 6th, and it took me awhile to get there because even slight uphill slopes would start slowing me down.
Cartegena hit it on the head, the steering becomes definitely more sensitive (although less so because of the progressive style steering). But I felt really confident because the car was still quite planted and I didn't feel the front wheels getting light or anything. Unlike other cars I've been in the Boxster just doesn't seem to lose its composure beyond a certain point, and just inspires you to keep going. It wasn't until I crossed a big seam in the freeway and went airborne for a fraction of a second that I realized it's not just the car I need to worry about, and that was all for the day.
Nowadays...maybe 100 but only when passing. It's hard to be in a hurry when your in this car
-David