I have a 271 formula with natural a 540ci, 1.5 od /1"shorty/ prop is a 28p can anybody tell me with knowage of this type of set up how much more mph could be gain on fresh water

speed decreases on fresh water because there is less buoyancy than there is in salt water...a boat is slower on a lake than it would be in the ocean.

Are you saying it is closed cooled?
If that is the case I don't believe you would gain any speed

If you are talking about removing closed cooling then there is no gain, no loss. You still will be running all the same pulleys, pumps..... You only loose a couple HP running all that stuff anyhow.

If you are talking about fresh water speeds vs. salt water speeds then most boats run the same speed. Yes salt water is more boyant but it is also more dense. Dense water is more drag.

you will 2 to3 mph faster in salt water, maybe moore dense, but less boat in water to drag because moore boyant