@Zee
what would be your budget? i ask because for some 100 or 200 bucks, or so ..........
did you ever had a view at ebay.com ?
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this here, a pre-owned full GEN1 akrapovic 4-1, would be my personal choice
even you have a gen2 - the gen1 system should fit - 99.5% of all dimension are similar
2007 Suzuki Hayabusa GSX1300R Full Exhaust System Mufflers Headers - Akrapovic | eBay
as far as i know,
this full system is up to 25-40 lbs lighter than the original - less weight causes always more acceleration
and
all the akra-systems (4in2 & 4in1) are known that they increase the power ~ 10-15 hp
without any other changes over the entire rpm-range (over ~4000 rpm til end).
and what ever kind of system you are thinking about
ask at very first the manufacturer for the hp & nm - diagramm taken with a comparable motor (~same cubics / ~ rom-range)
and if he doesn't have a diagram avoid the system !
as far as i learned, around 75% of all systems steel power and torque
if you don't want to invest a lot of money in an exact engine set-up.
i guess this will definitely take 3-4 (or even more) test runs on the bench - and how much will each of these test runs cost? thatby your budget will then be massively burdened guaranteed.
my tip

- if the money is not enough at the moment - keep saving and then buy something really good for a bit more money.