What would you suggest doing with a stock 2003 STR? I'm thinking custom intake, high flow cats, x pipe and 3# pulley.

 

The factory center muffler is kind of like an xpipe it's a sheared chamber ,don't think you would achive anything Buy having both , mine responded well to all that you listed plus cat back 2.5" xpipe replacement for the center muffler & two oval step throu chambers at the rear,
But I have not done high flow cats as yet . It's defiantly quicker than when I bought it.
I'm still waiting on the mina cold air devider , and cats to come later !
If If your looking for big power talk to avos about his twin screw supercharger kit ,
You might need to add to that return thou !

 

Caldoofy intake pipe with K&N, looks great, sounds good and a slight HP gain