I did some more driving today and i think at FE1R there will be possible to achieve nearly same laptimes as XFR or maybe even UFR WR times.
Also this is a hard situation because XFR and UFR in your league are restricted so they are even slower by this porbably around 0.3s +/-. Anyways nGT's has better acceleration so it will be disturbing to make a good run for GTR's.
From other side GTR's has so much grip and comparing to nGTs its faster like 10km/h to even around 20km/h per turn. GTR driver only has to keep highest possible speed and holding throttle pedal for 80%/90% of lap but a nGT driver has to be just an alien in his car to achive GTR's pace. Also Im worried it could make problems when comes to turns:
xr4 can go in T1 on fe1r like mad from 190km/h and just lift throttle and put full again into apex without losing any traction and coming thru T1 with minimum speed of 150km/h next turn is possible around 140km/h, comparing this to even non restricted nGT its like:
slowing hardly from 200kmh (in restricted is 190 but it doesnt matter) and going into T1 with 140km/h with really really hard work. I think I've tryied to make quick enough T1 in XFR like 10-15 times (yes that much times i was out from track after t1) before I learned how to drive it "quick" and still was 15km/h slower than GTR.
Sorry for my text Wall to you but in short: im not sure how it will be at fast tracks but on FE1R it looks like one will just disturb other to make a clean lap without contact (in that slow and easy to drive cars like GTR any contact can ruin whole race).
Im not going to make your league down or sth, I've just did some Testing for you
.
Just think about it, test more and how you will make it is up to you