The stinger is the small diameter pipe between the expansion chamber and the silencer - sorry im not tryin to be a smart-arse i jus wanna clarify the bit im on about!
Anyroad - if you look at a set of tyga exhausts you can see that the stinger on the left hand side exhaust is alot longer than the right hand side one. Its not just a case of how long the stinger part of the exhaust is but more about its total tuned length and the sizes and pitches of the cones in the expansion chamber part - which gives you the characteristics of your power curve.
On the MZ race bike we were running at uni, the first exhaust we made had a longer front cone with a shallower pitch, no straight section and a much shorter, steeply pitched reverse cone. I don't think we paid much attention to the stinger length or the length of the silencer, we were just copying other bikes we'd taken photos of at previous races. That exhaust became the benchmark for us and we ran it for three races.
The second exhaust had a much shorter, steeper front cone, no straight section and the same reverse cone and stinger as before. The silencer was the same length but much bigger in diameter because we'd been moaned at by scrutineering for being too loud. Overall the exhaust was shorter but we were only interested in the expansion chamber's profile (becuase we didnt know any better). We had also chopped the header pipe down as much as the rules allowed.
This exhaust had the affect of letting the engine rev higher and it made significantly more power in this new, higher rev range. It also had a pronounced 'power band' whereas previously power had felt linear.
The Burwins bike (arguably the fastest and most developed) had a MUCH shorter exhaust, with a fat, stubby silencer and virtually no stinger section. So he'd found by years of trial and error that that total length gave good power, or perhaps it just gave the characteristics he was after as a rider.
Your MC18's exhaust will be sized and shaped to give certain characteristics, so changing JUST the length of the stinger (and so, the total length) will spoil that design's performance. By how much i don't know.
_________________
After years of moaning about immigrants now i am one...