wow, this has gone in about every possible direction =)

peak numbers have little to do with intake/exhause performance unless you remove a restriction. ie saying stuff like 'adds x hp or tq' are pretty dumb when optimizing and building up a system. if u go /w a 2.5" mandrel bent catback, the gains are worth it if you already have a CAI, downpipe and piggy (depending on ur stance /w the no cat route). that should be ur mod list if u dont plan on turboing any time soon and want the most bang for ur buck each step of the way. this will take care of enging breathing in before breathing out. freeing up exhaust bottlenecks doesnt help when you still have intake bottlenecks =)

but if u care about performance, dont waste ur cash on a lame muffler and just weld it onto the stock pipe...

 

dont forget the RRM intake manifold.. that'll help that setup.

 

Not to nick-pick, but that is not entirely true. If the stock exhaust is more restrictive then the stock intake, then you will see better results addressing the exhaust end first. Having said that, the RA is probably more intake restrictive then exhaust, so the CAI is where everyone seems to start.
Spending huge money to get so little, if any, improvements seems silly to me. Like blk-majik said, CAI, downpipe, and piggy is a good order to go. The only thing I would recommend though is to throw a Magnaflow muffler (#14829- they are a perfect fit, love mine), or the RRM axleback, on the rear before the downpipe, so the car doesnt sound like absolute crap.