I'm using the 15m Speed3 with a 3 fin race board. Upwind is awesome but you are right, you have to keep board speed and line tension up without letting chops or waves slow you down. With a twin tip you might try letting your knees absorb the chop and then pushing down on the back sides of the chops to tense the lines.
Your best bet to go downwind on a twintip is to try for big air. With the 17m Speed 1 on flat water I could downloop the kite into the powerzone and then edge against it like I was riding upwind and go extremely fast downwind.
I am having mixed luck going downwind with speed 3 especially on the raceboard. When I lose line tension, I just fall right into the water, and it takes a minute to build it up again.
yeah i do all the absorb the chop push down the down side thing. Yes i also loop downwind. which is fine unless your on a wave and the wave is pushing you faster than the loop..... then its tack upwind or have an expensive pile of fabric get washing machined in the surf. The other day it was a strange situation were i was over powered but still couldn't easily jump because of the wind/swell/chop combo. Well i could jump but becuase i didn't have the board speed i'd just stall and splash down.