Be very skeptical of any sizeable hp increase due to a slip on alone. .....if you do the "standard" mods of a KLX jet kit, snorkle-ectomy, L-mod, and a decent silencer you may be able to get that 10%.....which is about 3hp on a KLR. Of course you can probably get half that without the silencer. To get any more power you are looking at a 685 kit, then head work, perhaps cams, etc. etc......but it quickly becomes an exercise of diminishing returns. If you are looking for a significant power increase over a stock KLR, the best way to to replace the bike with a KTM690R. ;-)
My standard silencer post;
A subjective topic but to recap recent discussions;
I don't like loud but I've replaced the stock silencers on both KLR's for several reasons, namely; lighter weight (5 - 7 lbs), better tone, better looks. .....I wouldn't recommend it thinking there will be a noticeable increase in power, nor would I put much faith in manufacturer's that claim much in the way of HP increases (with no other changes).
After much research and some testing, I like the LeoVince X3 which is relatively quiet at 94db, doesn't require repacking (the only one that doesn't AFAIK
, except for the ridiculously expensive Staintune out of Austrailia), looks decent and has a spark arrestor and USFS compliance. Runner up would be the Lexx with the optional trail insert; also around 94db but it does require periodic repacking. I also like the FMF Q4 but it is a bit louder (though still trail compliant) at 96db (keep in mind that the increase in decibels isn't linear).
There are a great many aftermarket silencers on the market that are far too loud IMO and you'd do best to avoid them.