The GameSpot.com to GiantBomb.com migration was/is a similar situation. Many of GameSpot's top editors left around the 'Gerstmann Incident' to form Giant Bomb. It hasn't had a long term effect on GameSpot's traffic, and despite great content, GiantBomb hasn't really taken off[1].
My feeling is that for established media style sites, unless the people breaking away are doing something _very_ unique, it's the URL that's more important.
[1] I'm not privy to their data and goals, but traffic wise they aren't getting closer to GameSpot. It's just my impression.
The traffic isn't all that relevant. It's what you can do with that traffic. GiantBomb is where the content is. Sort of like Digg may have the traffic, but places like HN have the conversation.
For example, GiantBomb has something like 6,000 (at least) members willing to pay $50-$60/yr to subscribe to the site. That's fairly significant for a two year old site.
I do not think it needs much more "taking off". The site has 9300 paying members (according to [1]), which I assume means they're profitable. I get the impression that the founders aren't looking for more than a lifestyle business.
My feeling is that for established media style sites, unless the people breaking away are doing something _very_ unique, it's the URL that's more important.
[1] I'm not privy to their data and goals, but traffic wise they aren't getting closer to GameSpot. It's just my impression.