opinions

I've got the ASP pulley. I can't tell u how much it added, did lots of things at once. But you will need and alternator overdrive pulley too, and a smaller belt. ASP sells both pullies together.
 
headers are good. Few local friends run them with no issues.

IMO there is plenty of other things id do before buying that pully. Also as said other pullys are needed with it if you want it all to act right.
 
pulley usually adds around 3-5 typically, and some say upwards of 10-12, you'll only notice it on a dyno with numbers. Also, look at scoggin dickey, I bought my pulley for my car there and they were $100 cheaper than other places.

Cal speed and Speed engineering have cheap headers too, and stainless steel

CalSpeed & Performance Stainless Steel 1-3/4" Long Tube Headers with 3" Stainless Y-pipe for 99-08 Silverado
Speed Engineering 25-1012 - Speed Engineering Truck & SUV Longtube Headers (1999-2013) | Speed Engineering
 
Thanks for the input guys. I got the Texas speed headers and I think I'm going to hold out on the pully for a little while.