My experience with EBS was the the performance was decent most of the time, but would brown out intermittently for reasons beyond your control.
Keep in mind this is purely research and it may (or may not) have already been implemented as part of their production systems.
The virtual machine lies on EBS, but it is really fast to start and performance is not limited by EBS at all in our case.
that said, netflix seems to have solved the IO issue with ec2. I'd be curious to know more about their findings.
For cache throughput, our cache library adds zone affinity to memcache, so the clients are hitting the cache in the same zone first.
Also, we're now exploring their SSD options (http://techblog.netflix.com/2012/07/benchmarking-high-perfor...)