Well there is alot of speculation on this issue, some say its worth it, some say its not. Personally I think it could do nothing but help.
As far as exhaust first off, go with long tubes no matter what, shorties jsut dont add too much. go with bellangers or Mopar, just the bellangers are allready coated i think and Mopar isnt.
Go with a good mid pipe, and not cats or hi flos if needed. X-metal now has mid pipes 3 " with or witout cats, jsut check his web site
www.x-metal.com I think and get his number and ask for Gary.
Then pick your cat back, mot I have seen is the magnaflow or the borla, they seeem to be the most popular.
Have I seen back to back dynos, with and without headers on a paxton , no, so I cant truthfully tell youthey will help or hurt.
I do know HEAT is the major problem with the paxton, that and tuning, and also make sure to use the pulley supplied with the kit and not the stock one, you WILL get belt slippage.
Once again x-metal and DC performance have made major strides with the heat soak issue with these trucks and paxtons.
Most of the big guns I should say, are around the 650rwhp mark, with stock internals.
but to get answers on what you need I would suggest contacting Chris at DC perfromance or Gary at X-metal they should be able to get you what you need, just remember it costs to get what you want , if anyone says its such and such $'s, then double it and you should be safe.
Good luck bo, hope this helped.