In much the same way as the MKV was massively better than the MKIV chassis, the RS250 is over the R26.R.
The problem with the R26.R was the shell was essentially made of jelly (compared to the RS250, it's still better than most in its class of the time), so although it had fantastic underpinnings, it was compromised.
VW increased stiffness by something stupid like 60% iirc over the MKIV with new laser welding and build techniques. The Megane is the same, it's amazing the difference a stiff shell makes on a car, which is why race cars are seam welded.
I don't doubt for a second that Renault played with the geo etc to maximise their chances, but there won't have been anything different on the car that you won't find on yours off the shelf and it's nothing you can't get done down your local geo shop for £100.
Horst von Saurma got the RS250 around in 8.29. More power, which will make a massive difference on the long drag up Kesselchen, different tyres (Bridgestone vs MPS2) and a geo tweak? I'd wager he'd get the Trophy within 10 seconds.