With piping that large, your exhaust is stagnating in the piping. That will result in extreme loss of torque, which you are obviously experiencing.

A 300 whp turbo GSR will be fine with 3 inch piping, but any N/A setup will greatly suffer from anything that big. Since I don't know the other mods you have, I suggest you put a 60mm piping catback system on to regain exhaust flow and torque.

Bigger is not always better in this case.