Yep, we were out in it too Friday. Small Craft Advisories posted, so we stayed very close to shore, but hardly saw 15mph mostly under 10mph (no white caps). Usually the forecast around here is pretty accurate but I guess they had an off day.
To hit a steady 7-8k (9-10mph), you probably need closer to 20mph depending on your point of sail. The AI seems fastest on the beam or broad reach (across to slightly downwind). That seems to work best for us here as the waves (4-8') then don't slow us down if we're running along them. A steady 7-8mph is more easily doable especially if the waves aren't slowing you down.
Best speed runs I've had were running along the top of long waves (1000's of feet long) but they don't usually form till closer to 25mph (still not usually a Small Craft Advisory here
). We don't usually go looking for conditions like this but have to go through them to get back home sometimes (and then an extra run or two out and back, just to be sure).
Also, the amount of reefing of the sail makes a difference. Once the leeward ama starts burying, the speed drops a lot. Often with less sail, we go faster (less drag).
A nice plus on using a spray skirts, when the ama "submarines" from a big gust common at faster wind speeds, the sprayskirt plains! Sometimes causing us to go even faster depending on the gust. Also buys us time to re-adjust to the changed condition instead of heeling way over.