Welcome to the forum,
ptgt2point4--great place to obtain information and exchange ideas about PT Cruisers. It's no surprise to me that you found the GT "an amazing ride" because it is a great car and lots of fun to drive. [8)] I'd recommend you start out with a drop-in K&N air filter and a free-flow (no baffles) performance exhaust. Do a search and you will find some good threads on the subject which will be of help to you. From there, the sky is the limit for MODS--be sure to hang onto your wallet.
Good luck. [?]