Just to add some fire to the DBR9's inclusion into Gr3 rather than Gr2...
The DBR9 was developed to two different specifications, one for general GT1 competition in the mid to late 2000s. Most sites and games put that one at around 625 HP and somewhere between 1100 to 1150 kg. On raw power-to-weight ratio, this alone should be grounds for inclusion in Gr2.
As
@Suzumiya pointed out, GT500 cars from the DBR9's era use steel brakes,
not the carbon discs allowed in FIA GT1, and only switched up when they went to the Class 1 rules. GT1s also only have 280 or 290 mm wide front tires,
not the ultra wide 330 wide beasts found on the vast majority of GT500s from the era, thus worse tires too. It's true that the majority of 2000s era GT1s barring the Saleen S7-R and Maserati MC12 definitely don't have anywhere near as much downforce as any GT500 (there's a reason the 550 GTS that ran in GT500 in the early 2000s needed significant aerodynamic mods), but at Le Mans, Monza, or similar power circuits, the GT1s should be able to edge out their Japanese silhouette contemporaries.
However, there's reason to go the other way into Gr3 as well: handicapped further on worse spec tires, and with a curb weight closer to 1300 kg, the DBR9 of the 2010 and 2011 FIA GT1 seasons is only a second faster around Silverstone than a 2019 GT3 car.
Why can't we have both? A '10 DBR9 with its heavyweight handicap, and an upgraded version of the '08 car with period correct aero and weight, but a BOP that puts it at 680 to 700 HP, and modern racing tires? I love the GT500 cars just as much as any fan here, but surely I can't be alone in wanting more variety in Gr2?