Owner of a Gemini..
As a previous owner of a Gemini 105 MC, and reading your posts, I would say to look at these things first: (1) Make sure the prop is the factory prop from Sonic (in the UK). (2) Make sure the engine is maxing at its proper RPM, and if it is not start with the fuel system (Raycor filters, etc). (3) Then depending on the weight of the gear aboard could be causing more drag. If you find out the problem I would love to hear what is was.
On another note, how did the Gemini capsize in 2005? Would love to hear about that as well.
Capt. J.B., Marine Surveyor