My drive showers extend down 1 1/4 inches. They were this way when I purchased boat. I saw a thread a while back with recommended length. I experience cavitation coming on plane if I give it to much throttle.

Yeh, that's too far. I had one down that far and would cavitate the prop enough to blow it out in mid 60's mph.

You can usually run them right up to the cavitation plate.

I'd cut off about 1/2"

I had the same problem and slash cut the tube pretty close to the plate. It flows plenty as there's no shortage of water under the plate.