Hi Olivier, enjoy the winging, we had a good session in the waves here last night in Denmark, lots of fun. Hopefully this week some windsurfing too. Quite a few guys out here with Mamba´s and very good feedback. I'd say for 20+ knots you should be fine with the 84 actually, as it will perform better with the 4.2. The tails and noses are wider than classic shapes, so you can go out in pretty light winds, 5.3 is also a top size. We have tested the 84 and 94 with those exact three size sails mainly in Cape Town for the final phase.
The 94 will be fairly quick, of course the flatter rocker of the FW make it hard to beat in top end speed, for sure better control in stronger winds. if you said winds 15+ knots then 100% the 94. For over 20+ knots as a starter, then the 84? makes sense?