With a forced induction engine, the more intake and exhaust restrictions you can improve on, the more power you can make.
Drop your entire exhaust and see the power sky rocket......you just won't have much torque below 2500 rpm......and you'll need ear plugs.

how much do headers run? vs PEM'*?

So true. I had PEM'* and 3" striaght through from the manifold to the stock muffler (only a 3" HF resonator, no cat) and in the top end I felt much more power.
Yesterday I put my 3" HF Magnaflow cat back in and I gained a noticable amount back in the low-mid ranges, but up top, the more I take it there, the more I feel I've lost just by putting the cat back in.
I think it'* time for a cable operated cutout (closed for the launch, and then around 30mph yank it open and unleash the fury), after I get my fueling and tuning done.

I can only imagine what headers and Stage II or III cam would be like in a Series I.