one more thing i will add, the setups i'm using are the tarmac ones posted by Nigel Atkins.
Are these the guidelines you are using?
http://www.racedepartment.com/forum/threads/good-base-settings-all-cars-all-surfaces.30193/
If so, these are pretty good from my experience.
If you are getting understeering, it may be because of the balance between your wheel rotation and the max Steering lock (I don't know what setting you are using there).
For a G25 with 360 deg rotation, you might need about 600 to 700 for the max Steering lock. It depends how much you like to throw the wheel. I don't like it too twitchy for tarmac, so I aim for more wheel rotation and higher steering lock.
I find that on tarmac, the turn-in and grip often depends on driving technique, I have to drive very differently to gravel or snow. I need to have plenty of power on through the turns, it usually grips better than you expect on RBR. If I try to turn in whilst off the power, it understeers. So, I brake to desired speed for entry, then get on the power as soon as possible through the turn.
If you are still having understeer, you may need to increase the rear ride height compared to the front, reduce the front AR bars, reduce the front suspension settings (mainly springs). I wouldn't recommend moving the brake bias too much to the rear on tarmac, it can make things very unstable, especially if you are trail braking or left foot braking.
Sorry I don't have any experience with the URT, but it should be similar to the other Group N cars I have driven.