My advice would be to go for the futura because it was really build for blasting even though the GO has a price advantage and still gives you the excellent blasting skills you want from a board. I'll leave that issue to Roger.
As for planning threshold, i don't know how wide the FF198 was so it's hard to tell but i'm guessing they'll be comparable.
If it's easier to controle an 8.5 in 18 knots is a question of training. I sail my 7.6 no cam freerace sail up to 20 knots without losing too much controle on my GO 139 with it's stock fin or on my 113 L board with a select Scross. Up to a year and a half ago that would have been impossible to immagine but a good deal in weightgain and some powertraining got me there without increasing my TOW dramatically (which is unfortunate).
You're a heavy huy, like me and if you're buying fullcambered sails like i suspect you are you can easily close a 1.3 mē gap. Personally i go from 7.6 strait down to 6.2 (i'm 100 K wet weight) and then down to 5.4, but usually, in one session if i have to change my 6.2 i usually go strait down to 4.7. When i start a session in 5.4 and the wind picks up i can usually go down to 4.2 at once so i think a 6.5 or 6.8 full cambered sail could easily do the trick.
Haiko, AKA crazychemical