RE: HPI RS4 Upgrades...
I'd recommend the carbon fiber upper deck and shock towers. I would advise against aluminum parts because despite looking nice, they add weight and don't really benefit performance much. In a crash they also won't fare very well, ending up bent where plastic would have either flexed and rebounded or broken and called for an inexpensive replacement.
As for drivetrain parts, I'd recommend a racing clutch and fiber brake to start.