American muscle vs japanese ingenuity

[AG4]

RE: Z06 - should be 400hp, GT4 still mistakenly uses the earlier 375-385 hp motor (without/with oil change), when the Z06 of that year should have near 400 hp.

The Z06 in GT4 was the earliest one, the 2000 model(actually its for the 2001MY). Its engine was rated at 385hp.
It was in 2002 that the C5 Corvette Z06's engine was upgraded to 405hp.