i think 10-15 knots is plenty to have a 150 go cranking. however to crank upwind with max speed a 70cm fin would definately help out.
a couple of other things to look at -
boom height - setting a high boom, around nose height is good.
sail outhaul - setting the sail really full as per speed sailors is not good, try more outhaul.
harness lines - run long harness lines and hike out. drive through the fin and mastfoot while swinging your body forward. (ie. straight back leg). this gets your body weight off the board and transfers max power through the rig.
rig position - try and keep the rig as upright as possible.
board trim - try and keep the board level with just a "small" amount of leeward rail bias. (railing the board)
wind - you need to read the wind. head upwind if the wind shifts then downwind slightly if you start losing board speed.
water conditions - head upwind more when sailing down the back of the swell.
for formula, 10-15 knots is definately enough wind to get a big board flying and pointing high. on the race course though the guys on the 12m sails would out point an 8.5 easily.
upwind a formula board can't match a mono but slightly less angle and more speed will probably see you to the top mark first. put a formula board up against an 18' cat and it's game on.

i reckon that would be a close race. it's fun sailing formula against cats.
that said technique is a very big part of it. any of the top slalom sailors on small gear could beat the majority of us on big around a triangle course. mainly through better techinque that produces faster board speed.
i wouldn't compare formula or "big" slalom kit to longboards though. the ride on a longboard is cruisey. even in "higher" winds. formula is more on the edge flat out sailing.
i had a go of a Kona on the weekend with a 6.2 in 5-18 knots of wind and it was a ball, felt like a raceboard without with the tracking and more of a carvey feel. for very light winds the raceboard and one design would be faster. but big thumbs up for the Kona!

one-design will do anything,

but doesn't feel as small as a kona. and lacks the top speed of all of the other types of boards. but it will do anything.

Div 2 boards - do you like a challenge

raceboards - technical to sail, sliding mast track etc. feel like half way between formula and Kona.
formula - will sail fast on any point of sail. more slalom board feeling.