Tzur- I would get a bigger sail for the light wind stuff. That will be the best way to improve your skills and get more time planing.
I think that Crazychemical's estimate of how much wind it takes for an 87 kg person to plane with a 7.0 is a little low. My "sail size calculator" says that 17 knots is the windstrength needed for an 87 kg person with a 7.0. Check it out-
The calculator also says that with a 9.8, you could plane in 12 knots. That would be a big improvement. It would mean you could plane more easily in the lake, where the water is flatter and better for the GO.
I think your strategy of riding the Calypso with the 7.0 when the wind and waves are very strong (more than 20 knots), or very light (less than 12 knots), is a good one. In summary, this is what I would do-
5-10 knots wind: Bic Calypso and 7.0 sail (non-planing)
10-16 knots wind: Go 180 and 9.8 sail (planing)
16-20 knots wind: Go 180 and 7.0 sail (planing)
20+ knots wind: Bic Calypso and 7.0 sail (planing)
The skills you should work on are getting on a plane, getting in the footstraps, and gradually moving the footstraps to the outboard and back positions.