I'm going to question just how have you determined that XRT needs 1% restriction while FXO is having only 3%?
I remember
Gener_AL (UK) and myself testing XRT vs FXO on BL1 a little while ago and I beat him with 5% restriction in FXO whilst he was in XRT in something like a 10 lap races!
I see you're going for a concept of a bit longer races here, but I assure you XRT is not a baby on tires. I have huge experience with that car and it heats them just as FXO and RB4 does. It just doesn't heat fronts more than rears like those two cars do, but instead it heats all four tires to about 80C, when anything over 70C is really loosing you grip!
I see you want to make RB4 just as competitive because there are Blackwood, Aston and Kyoto in the mix with their striaghts, so 1% could be justified on XRT.
Now we had been racing TBO's in
Dailycious series and FXO was dominant with 3% restriction while XRT was having none, even on tracks like Kyoto National with that huge oval straight!
We, the ones that prefer RWD for it's challenge tried once again to beat 3% rest. FXO with XRT, this time on South City Long in Apex cup. You can take a look in
here how that go.
So the conclusion is FXO needs 4% air restriction while XRT is having none to make both cars equal!
I presume you won't be giving up on your idea of restricting XRT with 1%, so I would suggest you to increase FXO restriction from current 3% to 5% air intake, for simple reason of having all three cars equal chance of winning any race, as championship overall!