I had the dual mufflers on before the header, it was much louder after the header, probably mostly because I removed the first (small) cat. Drone was pretty bad too (BTW these were original 40's I installed, they didn't have super 40's or delta 40's at that time). I only really saw a change at high RPM's, it pulled better there.
TBH it wasn't really worth all the money and effort I put in to install the header, the n/a 2.3L just doesn't make any power with bolt-ons. I did like the sound though.
2005 Mustang GT
CMS Stage 1 cams, Lito Tune, C&L Racer, CMDP, SR UDP, JBA LT's & catted H, FRPP Touring AB, Koni STR.T, BMR springs, J&M upper/mount, BMR lowers & brackets, J&M pb, Steeda Tri-Ax, Brembos
2009 Ranger XL SCab-4.0, auto, 4WD