Well, I'm glad you checked the obvious. You asked the right questions.
I don't have a Gemini, but my boat is very similar and I have been around them. 4.9 knots is very slow. That is only about 1/4 horsepower (I can reach that with one engine at ~ 1/2) if everything is right. My top speed is about the same - 7.2 knots with 2 x 9.9 hp.
Under propping seems absurd on a late-model boat... but I don't blame the poster one bit, because those are the things you check when nothing else makes sense. Odd things happen.
Do check the boards carefully. You should be able to hit 7.2 knots boards down - mine don't come up! Additionally, his reason for leaving them down was very lame. The reason folks leave them down is generally because they are jammed and possibly laminated. The factory recommends leaving them up EVERY TIME. They may also be wood and ruined. Check the Gemini site (Yahoo) for opinions, but this is real $$$ or really big DIY effort.
See the picture of me pulling a Seabrake drogue at 4.2 knots!!!!
Sail Delmarva: Drogue and Parachute Sea Anchor Testing: A Summary for Small to Medium Cruising Catamarans
. Sail Delmarva: The Merits of Learning to Sail on a Small Boat[/url]. It's GOT to be prop/engine issues.