While this was written in 2009, I don't think anything has changed in the intervening years. A 2.7 is a 2.7. Until it get to be a 3.6 or so, you aren't going to see a big difference.
https://sites.google.com/site/mikefocke2/itisn%27tfastenough will tell you all the things you can do to make it go faster. One important things is the money flees your wallet faster the faster you try to go. You can spend up to around $25k for the ultimate Boxster engine.
I've owned 5 sportscars with as little as 1.3 litres. They were fun because it was all about the driver and not the machine. I had a 2.5 Boxster and a 3.2 Boxster S and I actually preferred the 2.5 because I could keep my right foot pressed down harder and longer. I've driven Corvettes, they always felt ponderous. Just my preferences, I guess.