Understeer? On the RGT? Huh? Now the CTR2 understeers like a champ. You need to adjust your braking for the car. BUT! The RGT, CTR, BTR are all massive oversteer'ers (not ever a word I know). The 911/RR base, with all the weight in the back lends nicely to this.
The only understeer I can think of happens in the CTR/BTR due to the weight shift to the back of the car when on the gas which unweights the front wheels. So if you try to take a high speed corner fully on the throttle (i.e. Schwedenkreuz or Antoniusbuche on the Nordschelife, the 130r at Suzuka or even the final corner at Tsukuba) and aren't set up properly for it you will understeer off the circuit. Older RR Porsches/Rufs require a slower turn in, but you can get on the gas earlier once the backend comes around.
However, the RGT does not suffer from the above. It has pretty neutral handing.
Try going into the corner slower, 10-20kph (maybe more if it is a high speed corner) slower than you would with an FR. Let the back end start to swing out just a bit thus tweaking the apex of the corner for your car and then get on the gas. If you run your RGT against a similar power FR ghost you will see you gain alot of time out of the corner. Even against something like the all mighty Skyline R34 and it's mega AWD grip out of a corner...the RGT should hang with it.