To give you an idea of speed in the Hydro vs a flat. Jim Benson (Hot Blooded) was running 131 in his flat (River Racer record holder) in the 1980's. They pulled his motor out and put it in a new Fineline hydro and he went 150 on his 2nd run. I'm sure they went faster with some tuning of the boat/motor.
I think the flat is a better overall boat for lake use. The "set up" in a flat is all about getting the boat to set quickly and have the least amount of boat (drag) in the water for speed, yet stay alive while doing it. Balance. And it doesn't come easy, but that doesn't mean you have to do all the racers do.