I'd really be interested in your "my trims data" (GPS?) with a good 60 cm race fin.
I've found the smaller fin to be a good bit faster in very light winds with smaller sails (7.5 Severne Glide and 7.2 Sailworks NX slm), and the Serenity still goes upwind very well.
I've found the 70 cm fin that comes with the Serenity to be a bit "cathcie" (it loads one way, goes through neutral and then "'catches"very hard the other way) in transitions, and I've not felt that I was losing much upwing angle or speed with the smaller fin.
Try a smaller rig and fin and I think you will be even more stoked on the < 12 knot performance of the Serenity.
I've sailed it with a 5.6 m3 Sailworks Hucker and a 32 cm Lessacher Design or 32 cm Tangent Dynamics weed fin in < 4 knots and it still cruises right along.
Might try the mast foot more forward and try to keep the nose down (per Jim Drake) even when going faster as the board slips through the water better with the nose in the water.
Hope this helps,