Yes, that's what I was thinking.
The Hypersonic, (which is the board that Raggy is asking about) needs a little more fin span than the F2 Thommen and Tabou Manta (49 cm wide) because the Hypersonic is much wider.
You might get away with a fin smaller than the 28 on the Hypersonic, but you'd have to really stay "off" the fin, especially when getting the board to light up.
I just noticed that you are asking about the 6.6 and 5.6 m2 rigs on a Sonic 85.
For that board, I think Wolfgang's recommendations on the fins Dieter and Robert used would be applicable to your Sonic 85.
The Hypersonic, with a 7.5 m2 rig, would require at least the 28 cm Duo Weed and you could probably get even more speed with a 28-30 cm 15 deg. LE speed fin.
Hope this helps