First of all, Serres is not what I'd call a slow track. The best road cars reach more than 200 km/h on its main straights, and carry more than 145 in its fastest turn. Well, OK, compared to an F1 track, it's slower.
The R8 V10 Coupe was on OEM P Zeros, and it just did a couple of laps because it had to do a road trip with the same tyres afterwards...
The R8 V10 Spyder had Michelin PSS fitted specifically for a track test. Thus it did more laps, and managed to post a better time than the Coupe (which surely must seem a bit weird if you don't know these facts).
Now, about the Boxster: It wasn't Porsche that put Cup2 tyres on the car, but Michelin, which was the sponsor of that track test. We (Drive Magazine) always wanted to try a car on these tyres, so that was a great opportunity.