I currently run a 1/10 scale HPI Racing Nitro RS4 Subaru WRC 98.  Click the car below to view a larger image.

Modifications:

Thunder Tiger Racing Carbon Fiber Brake Disk:  Required a little Dremel work for a proper fit.  Braking is much improved, and it doesn't melt like the stock brake disc.  Outperforms HPI's fiber brake disc upgrade at a lower cost.

HPI Racing Carbon Fiber Front Shock Mount:  Provides additional strength for the front suspension as well as additional shock mounting points.

HPI Racing Front Belt Tensioner:  Required to keep the long drive belt properly tensioned.  Reduces slippage and extends the belt life considerably.

HPI Racing Side Belt Tensioner: Properly tensions side belt.

HPI Racing Heavy Duty Two-Speed Transmission:  Provides quicker acceleration and higher top speed (nearly 50 mph).

Dynamite R/C Sealed Bearing Kit:  Replaces the bronze bushings with sealed ball bearings in the hubs and differentials.  Quicker acceleration, lower friction, higher top speed.

MIP Nitro RS4 Mini Stinger Kit:  Included an aluminum header, gasket, muffler, and graphite rear shock tower.  Required extensive Dremel modification to the rear shock tower and header pipe to properly fit a Nitro RS4 chassis.  Well worth the effort, because it provides better performance than the HPI Racing header/muffler at a much lower cost:  Pipe exits out the rear (better chassis balance), and less prone to damage (HPI pipe is on the lower left side).  Exhaust note is much improved as well.  Note:  Locktite Blue or Red required to keep the header bolts from loosening.