I know it runs just fine, so it's ok for development which is great, but won't it have considerably worse disk and/or network IO performance because of AuFS ?
If you put Cassandra data on a volume, disk I/O performance will be exactly the same as outside of containers, since AUFS will be bypassed entirely.
And even if you don't use a volume, performance will be fine as long as you don't commit Cassandra data into a new image to run that image later. And even if you do that, performance will be affected only during the first writes on each file; after that, it will be native.
You will not see any different in Network I/O performance, unless your containers are dealing with 100s of Mb/s of network traffic and/or 1000s of connections per second. In that case, you can use tools like Pipework to assign MAC VLAN interfaces or even native physical interfaces to your containers.