The YFZ is pretty fast stock ... wait until you install a pipe. It'll want to rip your arms out of your sockets.
Before considering the purchase of a slip-on exhaust to upgrade your factory system, you will want to think about the type of riding you do and where you do it. You will want to factor in if you will be racing and if your riding areas have noise limitations. Starting in 2011, you will need to be 94db @ 3750 rpms in order to race Motokazie and AMA sanctioned events.
A slip-on exhaust and improved air intake are the first two modifications people make with their quad. I recommend that both be completed together, as they work with each other for best HP gains.
Can you just throw a new slip-on on your machine without doing anything else? Absolutely not. Most factory machines are jetted on the lean side to decrease emissions, and installing either a more free-flowing intake or exhaust is going to lean the mixture even more. Engine failure is imminent if you do not properly re-jet your machine.
The manufacturer of the exhaust is usually pretty good with providing you the correct jets for your application, but you'll still want to do a spark plug check afterwards to make sure you aren't too lean. Jetting is similar for all pipes, but it can differ greatly depending on where you live in the United States (elevation), and if you opt to run without an airbox lid or not. Some pipe manufacturers include a jet kit when you purchase their pipes. Otherwise, you can get individual jets and/or needles for the Keihin carb from your Yamaha dealer
For my location (1000 ft. ASL), with a K&N air filter/outerwear and open air box lid, I went with a 170 Main Jet, 45 Pilot, NCVQ on 3rd clip, and Idle Screw at about 2 turns out. I haven't had to make any additional changes. It was about spot on.
The NCVQ Needle that I went with is the stock needle in the YZ450F Dirt Bike (same motor). You should be able to ask for one from a Yamaha dealer. Do not buy a GYT-R jet kit, as it is basically useless once you go with the pipe (except for the pilot jet). All the main jet values are too lean. To figure out which spot is the "3rd clip", count from the blunt end down. You want to count from the end that has all the possible clip positions.
Rejetting your quad is something many people fear. Why not have a rejetting party and have a few friends bring their machines over to your garage? You can all share tools and lend each other hands when needed.
Here's a page that has videos on the bottom that I viewed before breaking into my carb.
http://www.allthingsmoto.com/forums/f-183/how-daddog-jetting-info-videos-12299/
This will give you a great introduction on what to expect when you rejet. One thing the videos fail to mention is that on the '07 YFZ450, the Airscrew is sealed with a metal plug for emissions reasons. I screwed a self-tapping screw into this plug a few threads, and then pulled it out. Then you can use a jewelers' flat-blade screwdriver to adjust your airscrew. (or replace with something like a Tusk or Zip-Tye Airscrew you can adjust with your fingers at will).
Putting the throttle cable back on the carb was the only thing that had me swearing like a sailor in my garage to my knowledge. My fingers are too big. It'd be easy if you were Eddie Van Halen.
Oh, and one more thing, use some loctite on the exhaust mounting bolts on the back, and keep them tight after every ride.
After the installation, to be absolutely sure of proper jetting, go give the quad hell for a little bit. Then, shut her down, pull the headlights, front fenders, and gas tank off, and pull the plug out and see what it looks like. Should be a nice tan color. White = Lean, Black = Rich.
