I own a single 300xs on my 21' Revolution.
Im in the same boat as yourself as i regularly race the boat however take my 1 year old daughter with us over to the islands of my hometown and regularly plod around at 20mph so as to not bash her around.
It doesnt harm the engine, however i try to open her up whenever the little one isnt in the boat so as to blow out any built up deposits from runnong at regular low rpm.
The 300xs is the only DFI engine by mercury recommending TCW3 instead of DFI oil. Apparently it has something to do with the fact that the TCW3 burns sufficiently at the high rpm where the motor was intended to be driven by mercury racing. However at lower rpm it would probably not be generating enough heat to burn the oil off properly.
Having said this i was told i could run DFI oil in it (its probably better for lower rpm running).
So far i have only ever run TCW3 and never had a problem.
Oil consumption depends on load and rpm.
Worst i seen (after run in) was 4 Litres of oil for 110 litres fuel. (heavy load, high rpm racing)
Best i seen was 2.5 Litres of oil for 200 Litres fuel (cruising around with the kids)