XF GTR and UF GTR are not from JGTC (or is it? tell me if I´m wrong). Those are just FWD class somewhere,
FXO GTR, XR GTR and FZ50 GTR = yes, they would be a JGTC
FXO GTR = Some Nissan (Not Skyline or Z or R390 anyway)
XR GTR = Pennzoil Nismo GT-R LM in my opinion but little changes of bhp&kg
FZ50 GTR = Some Porsche
As for the fantasy cars, the following is NOT an "official" list nor a "based on" list, just observations of what the cars may look like by several forum members.
Looking at the side view of the XR*, the door is slanted towards the rear instead of mostly straight, and lacks the covers of the Supra. Also the front, popup headlights are angled instead of straight.
IIRC the smallest Honda engine which is able to manage 100 bhp/l, was a 1.6 with VTEC on both sides.
The LX4 weighs about 460 kg (vs 430 of the Megabusa in racing trim), I wonder whether a Hayabusa engine can cope with that mass without modifications (a bike is actually half of that) - it'd be hardly surprising -if in a car- peak power is exchanged for a bit more low-end torque.
Exactly my point, since they never manufactured a 1.9L flat 4, they cannot step in and say, "hey that's my engine" - not that the Impreza looked anything like the FXO, but since the side is strikingly similar to that of the Astra Coupe, I guess they wanted to play it safe.
Well the biggest bore engine I know of that was in mass production was the Bandit 1200, I don't believe there are bigger engines that aren't custom. (I'm probobly wrong on this one though)