Back in the 80's we had a MkIIGR with an Evinrude 35hp and they performed very well together. We traded up to a MkIIIGr, and had plenty of ventilation with the same engine. Both boats were "short shaft" in those days. We tried numerous props but they made no difference. Interestingly, when we test drove an Evinrude 55hp "Heavy Duty" engine, the ventilation went away, so naturally, we traded up the engine, too. The added performance was really nice, but our main reason for getting the bigger (and heavier) motor was to get rid of ventilation.
Looking back on it, I think that the heavier engine just weighed down the back of the boat so that the motor sat deeper in the water. The boat certainly had a more 'nose up' attitude just sitting at the dock, and that angle would put the prop just a little bit deeper relative to the flow of water over the lower end. I really can't think of anything else to explain the improvement.
My WB 465 has HUGE buoyancy tube diameters. The ETEC 30 is one of the lightest engines out there in the relevant range. I wonder if a heavier engine would make the difference, again.
The ventilation is intermittent. It's fickle; very dependent on weight distribution. Over inflation of the keel is a must. It's also much less of a problem with more people (weight) in the boat. 90% of the time the boat runs well, but I never know when it's going to happen, and after 2 years of struggling with this issue, I am reaching my tolerance limit. I feel that this calibre of boat should not have this problem anymore.