I think the only way you'd notice a difference between the stock and aftermarket Doo is if your stock one, or the spring, is broken and your chain is stretched.
The replacement Doo isn't a performance item, but a replacement part that is MUCH stronger than the factory part, which has failed time and time again.
My buddy and I replaced ours, and both were fine. I then helped a few other people at a DooHickey replacement "gathering" (excuse to camp and drink beer) and 3 out of the 4 bikes had issues. 2 broken factory springs and 1 broken Doo.. Of the broken stuff, 1 was a 2006 and 1 was a 2007, so the dealer's BS about later bikes is complete bull.
The 2008 Doo is a different design, but from what I've been told, the spring's the same, and it's the reason for over half of the failures.
As mentioned, it's a $40 peace of mind issue.. Do it and you don't have to worry about it.
Also, you'll not find a guy who supports his product more than Eagle Mike. His phone number is in the kit and he answers ALL questions and will walk you through any issue.. Fantastic person!!!