I lost a stock tire the first week I had my xmods.
Anyhow, all better with the tire upgrade kit. I've since lost one more, and believe it or not, I actually totally wore out two soft slicks after massive amounts of runtime at a skate park (smooth un-finished concrete rawks) and tennis courts (smooth fine asphalt laid on top of concrete rawks even more!)
Anyhow, I took my Skyline's stock wheels and super glued a set of fresh slicks to them. Now it handles better, the tires don't fly off or come out of alignment and rub on things, and there's no phantom wheel spin (where the tire grips the surface but slips on the wheel)
Other than that, AWD is useless unless you have CRAPLOADS of torque (such as running a FET stack and upgraded motor and power systems) or you're racing on a really slick surface, such as ice (which is fun as hell with AWD) or hardwood floors that are not clean. In those instances AWD helps you handle better and accellerate faster. If you're not losing traction with RWD, then you don't need AWD.
If you're losing traction with RWD, try different tires and suspension setups before trying AWD. The AWD kit is worth it for the ball (limited slip) differential alone though, which I use on the back.
My current setup is fast as hell with handling to match for bone stock electronics and no "unethical" modifications whatsoever (only 4 batteries and official XMODS motors):
Stg 2 Drag (blue) Motor/gears
Stock wheels with soft slicks glued to them on all four corners
4 NiMH AAA's (Rayovac, not the 15 minute charge ones)
Ball Diff in back
Ball bearings
Yellow (medium) suspension in front, Blue (hard) suspension in back
Stock steering tie rod
I dremeled out some "extra" plastic on the body, like the parts under the hood, and thinned the plastic on some of the thicker areas of the body. I'm contemplating puttying over the spoiler holes and going wingless to save a few more grams
Also, I have pretty much every available part for the XMODS that RS sells aside from the different gearing for stage 1 motors and foam tires. Talk about a waste! I had all this stuff on my car and I'm like WHAT THE EFF IT'S AS SLOW AS IT WAS WHEN I BOUGHT IT!!!
Command Logic Lights, suck battery power and weigh 7 grams or so. OUT!
Body Kit parts weigh something like 12 grams over the stock parts they replace. Back to stock (I left the cool steering wheel alone though!)
AWD kit adds 6 grams or so, and increases drivetrain parasitic power loss without any speed benefit on most surfaces. Un-Installed!
If I were going to do it all over again, I would have only gotten the following:
1) Starter Kit
2) AWD Ball diff for $3 or so from atomicmods.com
3) Bearings
4) Wheel upgrade and Tire upgrade (and glue various tire types to the wheels)
5) Stage 2 Motors
6) Suspension
That's it. Although, I'm eventually going to be running 8.4V with a FET stack and upgraded motor, in which case I will have a need for the AWD kit again.