Sandy4Bama is right, the first thing is to get better clutching. You can't run faster if you can't get the power to the ground. Adam does some good work on the clutches on these machines. He has multiple levels of performance mods. 1-4 wouldn't be out of line for your XTZ.
Airdam Clutches - For People Who Want to Go Fast!
Second, I would consider getting rid of the OEM spike load dampner, I've seen these on several machines cause slippage in the drivetrain. Once the clutchpack in the SLD is fried, it'll never work right again. D&P Performance in Utah builds and sells the kit. $179.99 plus shipping
Spike Load Dampener Delete Kit from D & P PERFORMANCE
Third, I would install a Muzzy Dual Exhaust, the OEM exhaust is so restrictive and these big engines like to breathe and exhale lots of air. Some have had good luck with the Muzzy Fuel Controller, some haven't. I run and aftermarket pump (fuel cell) with a Trick Flow Regulator and set my pressure slightly above the 43psi that AC specs.
Next, I would put Maxxis Bighorn 2.0 on, they give good traction and allow more control.
Then I would look at installing either King Shocks, Fox Shocks or Walker Evans Shocks. They help keep the tires on the ground and allow you to stay on the throttle longer and harder.
If you want to get radical, either do it yourself or have a shop lower the seats 4 inches. This drastically changes the handling characteristics.
Now for the biggie, if you don't use it for dumping and heavy hauling, get rid of the dump bed. That thing is over 250lbs of dead weight. Build a lightweight bed and install your bedsides and fenders onto it. That will save you lots of weight and make your XTZ respond so much faster.
Do some looking around the site here and see what others have done for cage mods and such, there are some great things you can do that really improve the looks of the XTZ.
Hope this gives you a start point and helps...
Arctic Cat Prowler Desert Race XTZ: ITP BajaCross tires on ITP Beadlocks, Beard Super TZ Seats, Crowe 5pt harnesses, King Off-Road Racing Shocks, Custom Racing Chassis with 7 inch longer wheelbase, +5 Suspension set at 16 Front and 17 inches Rear inches travel, UTVINC did the tube bending
TEAM PROWLER NATION...