RE: Formula Boards
I'm not a member of the team, but I have ridden the 147 and own it's successor the 158.
Even though it's wide, the 147 is very nice in chop and winds; probably even more so than the 155. It blazes over everything at top speed without bumping because of the rocker design. It may even give you the confidence to stay with the 9.5 up to 20 mph.
Better upwind, better downwind, faster overall. 70 cm fin probably good for both sails. People usually don't use smaller than 60 cm fins on full-width formula boards, but the 58 would probably work if you are powered up on the 8.0.
The 147 may feel slightly more awkward when shlogging because of the short shape, but should plane just as early as the 155 when you get used to it.