Strangely, neither the embiggened friend list cap nor the "Finder tab" are present in our preview version of the upcoming Dashboard update. We're still locked in to the smaller, more intimate, friend limit. We're checking with Microsoft now to determine when this update will roll out to you and the masses of people waiting to be muted by you.
Update: It appears that the "Finder tab" feature refers to the "friend finder" on the Xbox Facebook site; not a part of the Dashboard.
Update 2: Major Nelson tweets,"We will not be expanding the friends list as a part of the next Xbox Live Update." That's not to say the friends list cap won't eventually be tweaked -- merely that it's not one of the changes included in the pending fall update.