The thing with WLP99, is that it works best when you start with a lower gravity, and build it up by gradually adding concentrated wort to increase the gravity. For example you would maybe make 3 gallons at 1.075, pitch the yeast, give it a day or two then add a half gallon to a gallon of super concentrated wort (around 1.13 or so), give it a day or two to eat that, then add more super concentrated wort, etc. Essentially you have to build up your beer over time. So I really don't see it working very well simply by pitching it into a beer that is already at 9-10%. If I were you I'd pitch something like 1028 or 1056, wait until that has done its job completely, then pitch a package of champagne yeast to drop those last few points of gravity.
Note: I don't have any personal experience with WLP099, the above statements are based upon my knowledge just from reading. I did however bottle a batch of RIS about a month ago where I pitched 1028 into it, then finished with champagne yeast. It already tasted great at bottling, I am going to let the bottles age for a few more months before I touch them, but just based upon my tastes at bottling time I know that it will taste awesome when I crack open that first bottle.