Yes, you can get hybrid cars with an Action Replay quite easily, as it works the same as a Gameshark.
The biggest horsepower I have seen? I made an F1 car have over 60,000 HP with the "multiply HP" code, but it was undriveable and a tap of the (X) button with the handbrake on caused the tach needle to disapear and the car to make the awesome tech noise, but it wouldnt turn so I just did the "get out of level" trick at the test course.
I have a Diablo that has around 1500 HP with F0/94 S (not sure on the syntax on the F1 nanmes, but you get the picture) Suspension, Tires, and Tranny, and FWIW it turns quite well.