In 8-12 "steady" conditions, the IS127 and IS137 could be good, I think.
But I definitely chose for the Ultrasonic just because the word "steady" is not compatible with our lakes next to the sea (e.g. Grevelingen & veerse meer) in these conditions. When I'm surfing on one of those 2 lakes, I always experience a lot of lulls and then I'm glad to have a board like the US147 that nicely planes trough it, points very high and always brings me back to my starting point.
It's different on the open sea, where you have the wind much more steady.
Oh and don't be afraid of the US147 when the wind picks up. It's rock stable and bloody fast. It really feels like a slalom board, much more than a Formula. I sold my F161 without any regrets.
Another advantage for me in very light wind is that I don't need to pump it onto the plane. But I suppose that's because I'm getting too old