the stock hole in the midrange is caused mainly by the std exhaust and the fueling , also some timing and secondary throttle valve settings,the std exhaust is actually smaller in the headers than the ports, everything after the headers is built to strangle the mtrs performance thru various rev ranges, well a cat converter is mainly for emissions, that plus the lean fueling, actually the 'muffler' doesnt actually retard any noise, it just modifys it, yes powerband can be altered via an exhaust, but really the optimin header will just accentuate the power demands of the camshaft,
with some race cams i tried the topend was much improved (dyno'd at 172rwhp), but accentuated the 4-6k dip, i really want a progressive delivery with more at topend, but not so torquy in the midrange, oh i have built the ultimate header for the stock cams, it really livens the power delivery from mid to top, maintains good drive ability at low rpm, but this makes for very hard use of tyres due to the wheelspinning, and powersliding !
for me the engineering challenge is nearly as interesting as the riding, well at least as long as im off the road due to injury, i have a tarmac hillclimb event to compete in this march coming, i rode my brothers sv1000 in it last year, it was awesome fun, but i was limited by it only having 120rwhp (ive piped it for him) and i was told not to hurt it or damage it, so had to ride with a reserve, but im looking forward to having near 50% more power and no restriction to slow me down, you can google the event " cliffhanger hillclimb" a local superbike champ has the record time (naturally)i was 8th in the 1000cc class last year