They don't like idling for too long and need the odd "blip" to keep them running. I'd imagine it would HAVE to be noisier than the 4 stroke. They're a refined enough unit at revs - I don't notice any big vibration at close to WOT. At low speeds, they're not overly loud.
The performance should improve, especially the time to plane (hole shot). However, the extra 4hp won't make a massive difference to your top speed, but might mean that you can plane with three, on calm water.
With a similar SIB, my max speed was 18kts, one up, sweet FA on board.
Go on - buy it - you know you want to!