6HP to 9.9HP - worth the effort?

I have a Monark 1648 that presently has a Johnson 6HP outboard. The motor runs great and with two full sized adults it will make 8.8 mph. While it is OK I really want something with more power. I live near and fish Kentucky Lake which can get scary when the weather or an idiot comes up. I have been looking for a 15 to 30 HP motor (boat is rated up to 30) and have found little. What I have found is many 9.9s. My question: Will a 9.9HP be that much of an improvement over the 6HP?

I have a 9.9 hp Honda 4-stroke on my Alumacraft 1648. I get 12 mph (GPS) with 2 people and 14 mph with just me.

A number of 2 stroke 9.9 outboards are easily converted to 15hp, so it very well may be worth your while to upgrade.