I just recently got my 12' aluminum boat back on the water.  It's a shallow V-hull that originally came with a Johnson 7.5.  When I first got the boat we used the 7.5 and it chugged along pretty good while wide open.  A few years back my cousin gave me a 1993 9.9 Merc.  He was using it on a zodiac but after an accident he discovered that almost half of the cavitation plate on 1 side was broken off.  Last weekend I put the 9.9 on and tooled around the lake at probably twice the speed of the 7.5!  It was nice.  I was expecting cavitation problems but all went well except for one thing.  At top speed the cavitation plate is only about 2 inches underwater!  It didn't affect speed or trim at all (I adjusted the trim after a few trial runs).  Does that mean the shaft isn't long enough?  It's a standard length shaft.  The cavitation plate sits about 2 inches below the bottom of the boat.  I know its getting water, there is a steady "pee stream" coming out of the side of the motor at all times.     What do ya think?

A chunk missing shouldn't bother a thing

Actually, it sounds like it's too deep in the water and you need to raise the motor a couple of inches.  The anti-cav plate should be running at same level as the water.  I can actually run higher but you have to be careful that you don't get it too high or the motor will suck air in the upper water inlet hole, causing it to drop water pressure.  

On your high performance bass boat, it never touches the water once the boat gets up to speed,  that's why they can run those big hydrofoils and not affect top speed.

Try it with it set approx even with the bottom of the boat.   One thing, if you get it high enough for the trim tab to be out of the water, you may notice and increase in steering pressure on the tiller handle.

Thanks Ben, that's good to hear.  I'll experiment with it this weekend....in between fishing spots.