RE: F-148 (05) - Fin size
You have the F-Type 148, right..... the one that's 96 cm wide?
It comes with a 56 cm Drake fin as stock.
Upgrading to only a 58 cm fin is not going to give you any appreciable increase in early planing.
I'd suggest something in the 60-66 cm range to get the best early planing, and use the stock fin with your smaller sails in higher winds.
A powerful 9.8 m2 like the GTX really needs a bigger fin in light winds.
Hope this helps,