As far as the sport machines... The 450's are pretty even stock for stock. Put the $30 Cherry Bomb from Yoshi and a pipe and it will leave a piped honda...BAD. Put a new exhaust intake and jetted properly it will be even again. No other machine responds to upgrades like TRX450. With a GT Thunder rear linkage, steering stabilizer and dialed in shocks, the 47" wide can take turns just as fast as the 49.5" wide LT-R.
In the 400 class, the EX is the ultimate trail machine. The EX has much better acceleration but the Z has better top speed. The Z is more track built and the EX is more trail built.
Start with the simple stuff. Open up the airbox lid, put a better flowing air filter on it. If you use K&N get an outerwear Filter. I prefer foam for the dusty riding i do. Put a full system exhaust, not just a slip on. Then you can change cams to a more agrressive one. Don't forget to jet. With that you could see a 15-20% gain in HP if you choose the right stuff.
Jetting is a must, The carb from the factory is set for sea level and stock exhaust and air filter. If you add a K&N air filter, your air into the combustion chamber is now 50% more than what it was. You have to add more fuel to keep the air/fuel ratio correct. Anytime you make ANY adjustment to the intake of exhaust system you have to rejet.
agree^^^ if you buy the dyno jet kit you can call them and tell them exactly what you did to your quad and the tech will tell you exactly what jet and needle setting to use also the instructions will tell you the same thing.