Thanks, Roger. I'll contact you privately too.
Was on vacation last week at a my family's lake place. Got in four days of sailing, which is a miracle for a midwest lake in the summer. I didn't get out on a single once, because everyone was so excited to try out the tandem.
We had a great experience. Most of the sailing was with my on the back with a 9.0 Retro sail, and my wife or daughter on front with a 5.0 Retro Ripper sail. We were in 10-14 knots most of the time, and were having great rides across the lake. The board jumps to third gear really easily. It took a little more wind to get into fourth: in third gear the front of the board would lift the front sailor six inches or so into the air (!), and we'd be going probably 15 mph. Just a little more wind and the board would flatten out and take off. I wasn't wearing a GPS, but based on "feel" from when I've worn a GPS on my start I'd guess we were going 22-24 mph in fourth gear. The ride was silky smooth, even over moderate waves, and my wife and daughter really enjoyed the experience of flying over the water without having to carry a monster sail. (My daughter said "this makes windsurfing a team sport"!)
I also sailed with my brother in similar wind. He carried a 7.4 meter O2 from the Loft, and I carried the 9.0 Retro. Even with two 200 pound men on the board, she still lit up and took off. We had an absolute blast tearing across the lake. I even sailed with my dad (he's 70, and carried the 5.0 Retro), and in the gusts the two of us were able to take off, too. A very fun way to share the speed and excitement of planing with other windsurfers!
However, I would be very careful about sailing that fast with less experienced sailors. (My dad, brother, wife, and daughter are all good solid sailors on their own, who don't generally plane, but seldom fall in the water or even drop a sail.) In schlogging mode the board is fine with a beginner (I tried with my brother-in-law, who was a good sport), but it is very scary to go fast when you don't trust the person in front to be able to handle his or her sail as the apparent wind comes up, or if you come across chop. At speed, that extra mast and boom could become dangerous!
Two minor questions about the board: (1) I don't have a clear idea when to use the drop-down centerboard. We used it all the time in schlogging mode, and often in third gear (because I didn't know the gust was coming). Very occasionally in fourth gear the big extra centerboard became a bit much to handle, but for the most part two sailors could muscle it down. (2) We didn't install the straps, so I didn't have a chance to try them, but they seem further back then I usually wanted my feet. Any experience with the straps? (I should say that it was surprising to me we could get the Gemini into fourth gear without the straps, and without it feeling scary most of the time. I think it's because the board is just so huge that it planed early and easy, and was very controllable even at high speed. On my single I would have been terrified at that speed out of the straps.)