The Gen IV's that were turning those fast quarters were all convertables, which are lighter than the Gen IV coupe. What are the fastest verified times for a Gen IV coupe?
In regards to the axle ratio and HP bump, the first gear ratio was made taller to compensate; the gen V still gets to 62 in 1st gear iirc, which is great for publishing 0-60 times, but not so great for drag racing; no improvement was made from gen IV to gen V.
Also, according to feedback from Nineball, the clutch is a weak link when using the Gen V as a drag racer; if you can't get the power to the wheels, you might as well not have the extra.
The Gen V also has wider wheels/tires than a Gen IV, which means a higher moment of inertia, which requires more torque to accelerate at the same rate.
Bottom line: the Gen V was not engineered for drag racing, and there were some compromises made because it was not engineered for said reasons. If you're really serious about drag racing anyways, who cares if your car's stock? Last time I checked, the cars never come from the factory with a driver either way, which makes more of a difference than a lot of things you guys are bellyaching about.