Haven't sailed the 17, but sailed the Carbon 20 here in Singapore a fair amount this month. We sailed with both boards down both up and downwind up until about 10-12 knots of breeze. Above that, windward board comes up upwind and you have to tack the boards. Downwind I'm not convinced where the transition to raising the boards is- might depend on chop. We have very flat water in Singapore. We didn't raise the windward board higher than the leeward downwind because you are flying the hull higher.

Now, the boards on the 17 are actually longer than one the 20, so all I said above could be completely wrong!