RE: IAN- S-Type 2006 Fin size
Yes, you can ride the 2006 ST126 with a 44cm but in my opinion it's really at the upper limit of fin size for this board/rider combination.
Something like the 44 will certainly help achieve a planing threshold, and will to some extent help on the upwind in marginal conditions, but throwing a lot of extra fin at the ST's does not result in such efficent tuning as on a harder railed board of similar dimensions, and the responsive ride that the ST delivers once it is planing is also heavily compromised by relatively small amounts of "over finning".
Certainly by going to a more upright race style outline, you will increase the efficency of the fin in marginal conditions for a given size/depth and that will help.
If you really are constantly in sub planing or less than well powered 8.0m V8 conditions, then maybe try a 44 (if you can loan one?) to see how you like it. Taking a 42 can help the bottom end (over the 39 raked freeride) but still allow more planing/top end/response.
For your weight and powerful sail, it might be a good idea to seek out a relatively stiff fin, maybe a bit more than the DrakePro, which gives good bottom end and medium range with bigger guys, but doesn't take powerful/heavy rider overloads as well as some other stiffer fins.
Cheers ~ Ian