At what scale have you found Performance and Size Limitations an issue? I've seen people leave due to cost before (not that I thought they were always correct), but not for the other two you listed.
You're looking at $2700/mo for the IO provisioning and $750/mo for the 6TB of storage. Double those if you want Multi-AZ. Then you get to price the server size which I imagine is one of the more expensive options if you need 30k/6TB.