So yes, we're more or less out of luck -- as long as Wintergrasp is extremely popular (and even Zarhym remembers the naysayers before the release with a smirk), there will always be a certain amount of lag in there. There are a few good suggestions floating around the comments thread -- one is that Wintergrasp should always be conquerable, which seems like it would keep down on the flood of people, though of course there'd be other issues if that were implemented.
At this point, Wintergrasp lag may just be something we have to live with. Eventually, you have to think the population will drop back down, and then those of us still in there will enjoy lag-free battles again. But Blizzard has taken on quite a goal trying to do non-instanced PvP full of vehicles and towers without any lag at all.