Definatly get the headers first. I would suggest getting high flow cats too. Headers w/off road = raspy...headers w/cats = nice n mellow. See how that sounds on the stock cat back...I think you'll be pleased with the sound. Still quiet at idle and mild eccelration, gets nice and throaty when you get on it. Then, if you dont like that sound...try a different muffler. You wont need bigger piping because you already have 2.5", which is enough for 600hp+
Bigger tubes really wont affect sound any.
You can get headers as cheap as $399 all the way up. Stainless Works, Kooks, ARH are popular (and pricey).......others, such as OBX, Stainless Works Stainless Power and TSP have lesser priced quality headers.
304 stainless is the way to go........the extra mile is getting them coated, which claimes to keep the heat in the header and not in the engine compartment.
For the big name headers....$1500 will prob only get you catted headers.....for the off brands, you could prolly do it all for the money.
I got catted OBX stainless for $569......may be made in China, but **** nice product. Easiest install I've ever had.
2010 Inferno Orange 2SS/RS M6, Black leather, Sun roof, OBX Headers, Halltech cai, Mike Norris tuned.
Avenging Orange, 34 years of not buying into the hype!