I have always wanted to take mine off and see.
I would assume that straight away it would be a little faster with less obstructions of the water flow.
I think you come into problems in two ways.
Sucking stuff up, like sticks, fish, small children, etc. that can ruin the impeller or worse.
And I think on turns, the water will not be directed into the intake and will just slide off to the side, so the ski would lose power on turns.
How does yours work on the turns?
They make some top-end aftermarket ones that are supposed to obstruct water flow less for better speed and accelleration, while still directing water into the intake on turns.
I doubt it would be all that noticeable unless you are a racer. Stock may be the easiest and cheapest way.
Try to find one from someone parting one out.