DP first. Cheaper too, I assume. You will get immediate gains from that. Cheapest HP you can add and it helps the car breath better. No need to mess with the rest of the exhaust, at least not for a while. The cat-back exhaust isn't that bad, esp if you like some quiet. Then once you have done whatever mods you want (I'd do TIP next, as the flattened stock TIP is as restrictive as the stock DP - keep improving the breathing), tune it once and get it all sorted in one hit.
No sense going back and forth through the tune shop or doing it yourself every time you make a change. Why tune it and then mess about with the entire configuration you just tuned for?
CP_e Standback & PNP; CP_e 3" SS Downpipe; Corksport FMIC with Top-mount K&N filter & OEM Ram CAI; Turbosmart BOV; Dashhawk; Prosport Boost Guage; JBR solid shift bushes; DBA 4000 Wiper-Slot front rotors; Hawk Ferro-Carbon HPS Street front brake pads (@ 69,000km); Sumitomo HTRZIII's in 225/45 x 18