xtr v-brakes are really good brakes, they work well in dry conditions. The real crux is when conditions become wet. Vbrakes will not work as good, even if you have a real good rim. They are on the other hand, much lighter and easier to maintain.
The deore discs are a good set that utilize the same caliper design as the hayyes mags. You will probably not want to use a rotor larger than 160mm on the Suntour fork, as larger rotors can cause failure in forks not designed for it. If Suntour gives you the O.K. to run a larger rotor go for it.
The performance levels will not be substantially different between the deore and the XTs, unless you are hefty, your bike is hefty and runs very heavy wheels, or you ride in wet downhill conditions exclusively. All other situations you will find the deores more than adequate.
The same goes for Vbrakes. LX, XT, and XTR level vbrakes all are real good, the XTRs of course are better materials and lighter and more durable, but unless as with the disc you ride wet, long downhills, where you continuously brake for the duration or you need absolute stop on the spot power then they will more than suffice too.
good luck
