This is incorrect. The suggested approach makes the game significantly more accurate adding 3 multiplications and an addition. This does not make the game noticeably slower. If you're concerned about 3 multiplications and an addition you're going to need to show me some profiler data that shows this as a specific bottle-neck in the game-logic.
No, you are talking about adding all of these operations per object in the game world.
It doesn't stop at gravity either. Virtually everything in the game world is moving via acceleration. If you were going to do this in a consistent manner you would need to do this for every single physics calculation for every single game world object.
You are correct that you would have to profile it to determine how much of an issue it would be but it sounds like potentially a lot of deadweight to add to the game. Especially since when games slow down it is often due to having a lot of objects in the gameworld simultaneously.
