It might end up more than 10%, depending on the clock speeds they pick. Like if they go from 2.2 to 2.5Ghz would be at least 25% faster, and even better than that in some applications.
And if they throw in a better GPU on top of that...
No, it wouldn't. Increasing the clock speed from 2.2 to 2.5 GHz isn't going to provide anything close to a 25% speed improvement for most apps.
My reason for wanting for the upgrade is the potential for better battery life. The penryn has already been shown to give you more mileage than the merom on the santa rosa chipset (http://www.anandtech.com/cpuchipsets/showdoc.aspx?i=3195). I am holding out for the better battery life and the 6M cache.
According to Anandtech, it's just a few percent. Enough that I'd prefer it if I were buying a new machine, but not enough that I'd lose sleep over it - or spend all my time whining about the new ones not being available yet.