First of all, I'll answer what I highlighted in bold in the second post. The IMSA GTLM class (if we talk about this class) is actually roughly equivalent to the GTE class, with the addition of some suitably modified GT3s (such as the BMW Z4 and M6). The performance of cars in this class is undoubtedly closer to that of the Gr. 3s in the game.
Speaking of BMWs, neither the M3 GT nor the Z4 would be well placed in Gr. 2: the former has never been a FIA GT1 car (but a GT2 according to the 2011 regulations), and the Z4 is even a GT3 .
Apart from that, in Gr. 2 there would be some chaos. I'll explain. Currently there are the GT500/SuperGTs, as well as the only modern DTM car in the game, together with the two GT1s according to the regulations in force until 1998 (Mercedes CLK-GTR and McLaren F1 "Longtail"). Among the cars you listed, I could agree for example on the addition of the Porsche 911 GT1, the Nissan R390 GT1 and the Toyota GT-One.
On the other cars you listed (Chevrolet Corvette C6.R and Chrysler/Dodge Viper GTS-R), a hint on their placement could come from the current placement of the Aston Martin DBR9: in the game it is placed in Gr. 3. They should probably be in Gr. 3 also the two above cars.
In Gr. 1, well... In my opinion, the thing that definitely needs to be done is to separate the Group C cars from the LMPs. The HyperCar class also deserves a place of its own, but currently there is only the Toyota GR010 in that class.
If I may give my suggestion... A possible solution would be to create "historic" groups. So for example you could redefine the classes:
- Gr. 1 "Historic": all Group C cars;
- Gr. 1 "Modern": the various LMPs, perhaps appropriately divided into Gr. 1 Hybrid and not;
- Gr. 2 "Historic": the old FIA GT1 cars of the late 90s (Mercedes CLK-GTR, McLaren F1 GTR "Longtail", Nissan R390 GT1, etc.);
- Gr. 2 "Modern": the current GT500/SuperGT, the latest proper DTM-spec cars (as the Audi RS5 DTM);
- Gr. 3 "Historic": FIA GT2 cars from the late 90s and GT1 from the early 2000s (Aston Martin DBR9, Chevrolet Corvette C6.R, Chrysler/Dodge Viper, etc.);
- Gr. 3 "Modern": the current GT3s, plus some cars similar in performance (such as the already mentioned BMW M3 GT '11);
- Gr. 4 "Historic": cars like the Alfa Romeo 155 V6 Ti, and perhaps others that could arrive (such as the Mercedes 190E);
- Gr. 4 "Modern": the current Gr. 4 BUTs with the exception of FF and AWD cars;
- Gr. TC (Touring Cars, obviously): this would be a completely new group, the current Gr. 4 FF would converge here, together with those currently AWD sedans that could easily be modified as 2WD.
But we all know that will probably never happen... Not before GT8 I'm afraid, and maybe not even.