Thats a great time with a 7.8 on the F101. My impressions were the Futura accellerated very quickly to 30knots. If you can find some nice flat water where you can sail broad, the 6.2 in 20-25 will be a great combo. I would guess you should be getting 35+, no worries.
You are a similar size to me, and I have found 5.4 to be seldom used. Mostly I sail in 8-25 knots. The odd day a little more. I use 8.4/6.7 80% of the time and 5.8 the rest. Going broad you certainly need more rag up, so even though the 6.7 sometimes gets a handfull on a reach, its just right going off......