I have tested C2 in 6,5 and 8,5 good low end, easy rigging, and good balance. Really nice sails.
I have used Overdrive 8,5 and Code Reds' for a year. It is super fast and stable slalom sail. A 7,5 is perfect size for the 105.
For going fast, take the overdrive, no doubt. Very close to full race sail performance, but more grunt and way easier handling/rigging. Take the right mast, blue or redline and a mast extension that fits to the pulley position.
Fastest sails on iSonic are 6,2-7,8 sizes I think, but it depends on your weight and conditions.
It can take up to 8,5, but it depends on, if you are a heavyweight or lightweight.