what size track are you riding on?
how good of a rider are you?
do you tend to look for the limits all the time?
how do you find crashing?
With the ease of availability of 12 x 2.15 rims these days you would be crazy to buy anything skinnier (that second link ducar posted are 1.85 wide)
The kenda wheelsets that Dhz and Maylands have are a good entry level wheel set and make it a breeze to change from dirt to motard and back again. If however you have a fair amount of riding experience and some big balls to push to the limits you will find the limits of the tyres, I guarantee you this. However good tyres are expensive and hard to find, also pointless if your suspension is rubbish or poorly setup.
Normally the kenda's will let go just before you start peg dragging or basically just as you get your knee down road style, this is however on a smaller track so on a fast track may let go before those lean angles.
If you have the money to spend, the well setup bike and the riding ability you could go for a good setup from square one. MSO have some high quality 12 x 2.15 wheels without tyres for about the same money as the dhz setup, whack some dunlop, bridgestone, sava or similar race tyres on and you can run with the fast guys without the tyres breaking away all the time.