You need consistency (distributed mvcc), row-level locking and scale-out architecture. Clustrix might be a better fit for the workload. Clustrix customer Twoo.com has 336 core deployment (168 master, 168 slave, 21 nodes each), they have millions of users and billions of transactions per day. Their application still thinks it's talking to a single MySQL database and they don't have a DBA and they have never thought about shard keys etc. We in the database industry should be solving these problems for you.
