Is it possible to have a stock stainless 28 4blade worked to give me another 400 to 600 rpm. If so who would be able to do this and is it worth it or should i just buy another prop.. thanks!!

Yes. Any good prop shop can do it. Labbing will give you approx 200rpm, and you can have an inch taken out of it, which should give you another 200.

BBlades can lab it and get another 300-400rpms out of it. Really, if you want more rpm than that, you should step down to a 26.

Griff is correct.
If you truely need 600 rpm, your better off to go down in casting pitch.
Otherwise, we can get you 400 rpm. I may advise you differently though once we had a chance to discusss your set up and usage goals.

Brett