I saw online a motorcycle shop that added only a Jardine slip on exhaust and they dyno-ed it at 34 hp before and 37 hp post. Also a gain of 3 hp. Does that sound right?
Short answer, "No."
Someone on ADVRider.com offers a prize (think it's $ 50) to anyone who dynos a modest rear-wheel horsepower increase, with mere carburetion and exhaust tuning; NO ONE HAS EVER COLLECTED.
A problem with trick exhaust horsepower curves, compared to STOCK exhausts, sometimes the aftermarket curves are LOWER, at certain strata . . .
And, what is your service life spectrum for your KLR? At maximum horsepower almost constantly, or a little beyond, approaching redline? Are you in COMPETITON riding? Answers to these questions may help you justify your hop-up, or . . . bring you to . . . I would say, "to your senses," but that's too judgmental; let me say to perhaps more conservative and realistic goals and objectives.
As Spec says, hop-up leads to a CHAIN of inter-related systems; and . . . you know about the weakest link. If you're going to coax more oomph out of the tractor engine, you're going to stress components not designed for the added loads; FORGET ABOUT static durability and reliability, as you attack the World Land Speed Record, or your hometown main street grand prix lowest-recorded-lap-time mark.
The modest modifications suggested, increasing displacement by 5 % (i.e., 685 kit), may bring about 5 % more peak horsepower and torque, without seriously compromising reliability (although, men say, for every stress there's a strain; "no free lunch").
Worth it to you? Only you can decide.
Then again, some forum members claim you will obtain additional horsepower, greater fuel economy, decreased oil consumption, enhanced engine compression braking, and improved ring seating by merely installing a check-valve in the crankcase breather hose (the "PCV Valve Mod").
Imagine, all these advantages for just the price of a PCV valve and a couple of hose clamps; MUCH more economical than an aftermarket muffler! Quieter, too!
FULL DISCLOSURE: I personally remain skeptical of the claimed advantages of the PCV valve mod, because . . . I know of no tangible, repeatable, measurements of any of them, beyond the accuracy, precision, and repeatability of the error budget of the measuring devices used (e.g., dynamometer), nor . . . has the MECHANISM whereby a check-valve works all this magic been explained to my understanding (no crankcase vacuum has ever been detected, although I've no idea how a vacuum could work these wonders, even if it existed).