2003 Chevy Trailblazer LT 4.2L Vortec 40K miles 3.42 axle

Hi Guys,
I have some money to put into this vehicle for a few aftermarket performance upgrades. It's a great vehicle with no problems; just want to coax a little more from the DOHC L6:
Want to put a 3" cat-con off the exhaust manifold, as it appears no one makes a 6cyl. Header for this application, then into a 3" muffler with dual 2 1/2" outlets & pipes the rest of the way. Don't know yet if there's a pre-cat or AIR on main cat. Have found both cat-con & muffler apps online from Flowmaster. Q: Will this eliminate too much back pressure? Does EGR need back pressure to function, like some older Fords? Do I even have EGR? If so, would 2 1/2" cat-con & muffler be more prudent to maintain some back pressure? Thanks for your advice. Joel