I would be extremely surprised if WoW has issues when lots of people are crammed in a small space due to their Oracle backend. More likely it is all the calculations that the serverside is performing. I know a few people who work on other MMOs and they keep everything in memory, even accounts for logged off users.
I'm curious: what you think (or know?) they are doing that is causing the database to be a limiting factor?
I didn't mean to imply that the bottleneck was due to Oracle. I was saying the exact same thing you are saying - it's due to memory or CPU limitations in their application server, which forces them to heavily shard.
If Redis can help them scale from the small shard to larger shards that support more players in an area, it might be interesting.