There is a range of speeds you should go at, and it depends on variables like the ones Duffy is asking. Most often, I've found myself around 10.8-11.1 most often. On my old boat, I had to stay pretty close to 10.7 as more speed dropped the wave down quickly.
You really have to play around with it, but you'd probably be safe looking in the range of 10.8-11.5, with better odds of finding the sweet spot on the lower end of that range. If you've been surfing at 9.8, I'm assuming you're probably running with less than 1000lbs above the stock ballast? Usually the less weight you have, the slower you can go.