On a planning V-Bottom powerboat are chines below the waterline actually useful?

Absolutely, engine trim has to be exhausted as the solution, before anything else, and as I say, lowering the engine slightly might be part of that, but you need a trim gauge, and finding the point of "blow-out", compare that with the manufacturer's specs, if it is letting go too easily, dropping the engine height is a possible option. Realistically, this is an open boat, and any water being taken aboard, and especially in cold conditions, contributes mightily to the impression of a "wet" boat.