The overpowering of the majority of these cars suggests that PD's downforce model doesn't work that well with drag (it was only a recent update that even factored that in), so in order to give the cars the more accurate laptimes and pace at Le Mans and in general they bumped up the power.
They give all these cars downforce figures but seemingly don't account for how draggy or slippery the cars are, which is why the Pagani LM can give the R10 Audi a good challenge...
Don't know if anyone has mentioned about the R91/92CP specs. According to the actual Japanese race car driver that drove it, in Best Motorings Fuji Fast video( I have the dvd's and check later to be sure), i believe he says the cars were turned up to 1100hp for that circuit.
GT5's aero modeling is notoriously poor, as downforce is severely overpowered, or at least drag is not modeled well: even at tracks like La Sarthe max DF is still the fastest way around which is completely unrealistic.
In the car descriptions, there are some of the lmp cars that don't say the accurate amount of power, maybe it might be lost in translation or it may be that the game itself does this to the cars, you are allowed to tune them with turbos and oil changes to get more power.