If my current prop slip is @ 17% and I figure out how to get it down to 12% will I gain that 5% in top speed?

You could actually gain more than 5% in the speed category.

Likewise, if the slip goes down and so does the rpm and then drag goes up, you may gain less than 5%.

All in all you want to get your slip lowered and most likely you will gain in the speed dept.

what is the formula for calculating prop slip???

here ya go. http://www.go-fast.com/Prop_Slip_Calculator.htm

thanks damitt