Why would 'top' (whatever that means) WoW guilds have an interest in transferring over, as a guild?
The 'top guild' activities presumably are RBG or 25-man raiding, which are both instanced and on a different scale to instanced pve/pvp in GW2.
That's not to say 'good' guilds shouldn't take the opportunity to try something different and could work very well but I think the glory seeking will be different. The impression I get so far is that the 'big glory' for guilds will be making a name for themselves in WvWvW.
Given the public nature of the larger scale action, I don't see that happening in PvE in the same way.
Even with that said, there's no reason to think that ex-WoW players will dominate, in numbers or otherwise. Most of the WoW players I know are getting pretty fired about MoP and worryingly most of them took the Diablo deal. The only guild I know considering moving is a peripatetic one, lately of SWTOR.