First, the bits on the engine are anti ventilation plates, not anti cavitation. They help prevent air from being entrained into the propeller from the surface. Cavitation is another matter altogether and only occurs on high speed heavily loaded propellers and can soon destroy your prop.
If a boat is properly designed and the engine is correctly installed, there is no need for them.
They can also help get a boat onto plane and/or help prevent porpoising if it needs that. but if a boat is well designed and set up, they will probably just slow it down a bit.
Another point is that the Doel fins put an extra strain on the vent plates which occasionally would result in a fracture of aforementioned plate. (ooh err Missus!)
Have you got problems or are the adverts talking to you, you could experiment by plasma cutting yourself a pait of small trim tabs, although on a 4.5 they would have to be small
Here it comes again, I don't stand a chance
Soul possession, Got me in a trance
Pullin' me back to you - Deja Voodoo