OK... Heres how I see it:

9.9 vs 15

Neither one is gonna plane off a v20.

So you will be moving thru the water as a displacement hull.

That means top speed for a v20 will be about 6 mph.

A 9.9 will do that unless heavy seas or a current.

I am familiar with evinrudes ... so a 9.9 and 15 hp rude have about the same displacement. That is where your torque or thrust power is coming from. So a 15 is not gonna give you any more power that you might need in seas and/or current. The 15 gets the extra 5 horsepower by spinning an extra 1000 rpms at high throttle. That ain't gonna happen as a v20 kicker.

If you want more torque or thrust from a 9.9 you should go with one of the Yamaha or Mercury high thrust motors. I am considering this, myself. You may even be better off with a Yamaha high thrust 8hp than a conventional 9.9 for use as a v20 kicker.

Chipde, I know some of your speedy fish like kings and wahoos like a fast trolled bait. Is 6mph gonna be fast enuff for them? Get a kicker, anyway.

Mac, I know those Great Lakes can get rough as an ocean, and even have tides. Do you ever get any significant currents?