Off road pipe/high flow cat depending on your states laws and then a tune. These are the only mods that make a noticeable difference, trust me i have tried all the boltons.
I put in a high-flow cat in my SVT, mainly because the OE cat was trashed, cracked and leaking...didn't notice any performance from the swap (seat of the pants dyno)...it's a pricey piece, so if you don't need one, skip it.
The tune made a big difference (thanks Tom!), AND I get an extra 100km out of a tank of gas, which matters just as much as performance in my book.
And working with Tom to dial it in is pretty cool too...getting a new tune file in your inbox, uploading it to the Predator and reflashing the car is 21st century shit right there!