What do you mean by the most out of your ski? With the impeller, depending on the pitch of it, you will either get a better hole shot and quickness off of the line or better top speed. With your ride plate and intake grate, that too depends on what you are trying to accomplish. Do you want better stability or do you want it to scoop up the water more for more power? What are you wanting to accomplish and then maybe the question can be answered a little more better.
If you don't want to effect top speed, then leave your impeller alone. It sounds like all you would need to do is change out your ride plate and intake grate. There is a really good one that would do good for what you want to do but the brand name is escaping me at the moment. I will try and research it and get back to you.