RE: F-type 138 VS Isonic133
I don't havean Isonic 133 in my demo fleet, so I can't answer your question on the 9.5 m2 rig, but it would seem like it would be alot better than the 8.5 m2 on the Isonic 105.
Yes, I think you will loose a little on the early planing end if you move from 88 cm wide to 80 cm wide, but you are fairly light so unless you sail on freshwater all the time, I think you could probably overcome this with technique.
But I'm puzzled.
Maybe you rig quite differently than I do, but a 7.5 works pretty well for me in a sloid 12 knots, and I weigh about 10 kg.more than you do.
Do you have another sail in the 7.0-8.0 m2 range that you could try on the Isonic 105? Maybe rig it pretty "fat" and use an adjustable outhaul to "slim it out" once you get planing.
In a solid 15 knots, I'm normally on the 6.6 m2 Hucker on the Isonic 101 or the Isonic 115 I had last year.
As far as getting more top speed from the FT-138, yes, I've used fins down in the 50 cm range (Deboichet Concept 50) with a 6.5 m2 rig and the board simply flew. It does sound like you are tailwalking on too much fin here. Also, when I got max. speed on the FT-138 I had the mast foot all the way back and the 50 cm fin on very flat water in around 18-20 knots on a 6.6 m2 race sail.
Yes, the Isonic 133 should be definitely faster than the FT-138.
Hope this helps,