I think your speedo was off if it said 148. Since the top speed of a GSR is 134mph. A lot of cars that arent totally geared toward performance have speedos that become less accurate at top speed.
I dont know what youve done but im assuming you have a couple intake/exhaust bolt ons. Even with those your not going to see that speed. Your car runs mid to low 15s stock at 90-93 mph. A 911 runs mid 13s to low 14s at 98-102 mph. Course these times vary from autos/year of car/and drivers.
I find it hard to believe... I think Id have to shoot myself if I ever lost to a near stock GSR in my slightly slower than a 911 LT1 camaro. When my z28 was stock of course.
But nice kill on the porcshe anyways.