I am in the marine industry and have never seen a foil do much good. Tabs would help but the cheapest is a prop.
What is your RPM at Wide Open Throttle. If you do not have a tach have a tech hook up a scan tool to see what it is. You will gain about 300 rpm going down one size.
Say your motor is rated at 5,000 RPM at WOT and you are only turning 4,600 RPM then you need to step down a pitch. Say you have a 19 pitch on it now get a 17 pitch.
As far as motors go I would not touch anything but a Suzuki, Yamaha, and a Honda in that order.
Problem is if you have a Merc now its going to be the cheapest route to go with Merc again initially. Controls, wiring, gauges, should or may all work again. Over the long run though I have found Merc to cost the most.
Chris