I would say go with the Access Port/intake combo before the cat back. I REALLY wanted a cat back for my car, but I decided to go with the AP and intake first. I was very pleased with the difference. Better power delivery compared to stock, it was like a different car.
I was able to acquire and trade some parts to get the Q300 eventually, and I'm super happy with that also. I'm just glad I went in the order I did.
BTW, I'm now on a Torqued Performance Stage 3 tune running 3" turbo back exhaust, SF intake, Grimmspeed EBCS, Perrin TMIC, and TurboSmart Kompact Plumb Back BPV (fully recirculating)....that's just the engine mods, and I said I was totally done modding after Cobb Stage 1.
Just know, if you begin modding you will become like a drug addict...using any and all means to get more mods.