It actually makes the intended 986hp, but 493hp is all you see because that's its combustion engine output.
That being said, the top speed physics for some Gr. 1 cars in GT7 are slower than in GT Sport for some strange reason.
The game does not account for combined hybrid output when it comes to hybrid prototypes, notably LMP1 and some LMVGT's (also even some road going VGT's such as the Genesis X Gran Berlinetta, where it has an 869hp combustion engine with 202hp from its hybrid systems, not to mention the Lamborghini VGT as well, producing 774hp from its combustion engine, with a 33hp hybrid system output.)
Every other hybrid in the game does represent total combined system output though.