
Cost is lower than OEM replacements and they weigh less. Also, in most 
instances you do not have to rejet the carburetors to get a little more 
performance. I replaced my OEM canister with an Ontario systems slip on.
The mid-range performance is better but I also notice a slight stumble at 
2-3K rpm. After talking to Kaz Yoshima, the developer and manufacturer of 
the slip on, he thinks I might need larger pilot, idle, jets. Was not suppose
to but maybe where I live makes a difference. Altitude, barometric pressure...
Anyway, besides that minor glitch, I like the slip on performance and the sound,
which is louder than stock, but not loud enough to annoy folks. The Ontario 
slip on use some sort of sound baffling technique to reduce the sound at full 
throttle.

Just remember, every bike is a tuned system. When you change the intake or
exhaust characteristics beyond the allowable delta of the manufacturer, you may
have to change other areas to compensate. Like a complete change of the exhaust
system, header and muffler, changes the exhaust characteristic to the point
which re-jetting is required to compensate for the new engine breathing/pumping
profile.

Talk with the folks who make the slip on you intend to use and ask them if
they have used it on your particular bike. Check with people who have the
system also to see what they think about there slip on.

Also, as a last data point, in Sport Rider's first issue, did a review on a
group of header and exhaust systems for the CBR600F2. They also included, as a 
side bar, the Ontario slip on, the Dyno showed an increase from stock of 5bhp at
the rear wheel, 85 verses 91. 

Buyer beware, of outrageous claimed increases in horse power! It may have been 
true in the past, but not today!