I would pitch 2 re-hydrated packets if you have them. You could do a small beer, and pitch the big ones on the cake, that works great.
Also, if you are using any simple sugars, add them after the fermentation starts to slow. Cant remember where I read it, but the theory goes something like this...
If you add the simple sugars (sucrose, corn sugar, candi sugar, etc.) the yeast will go for them first and may leave more of the more complex sugars unfermented. If you start them out on the tough stuff, then treat them to some easy work as they are slowing down, attenuation will be greater.
I just got a 1.090 tripel down to 1.008 in 7 using this method.
I am not sold on using Chanpagne yeast. I did a barleywine (1.096 OG) and it attenuated fine and carbonated fine using Nottingham. I guess that begs the question of what yeast you are using?