Having written some analytics services myself, I'm curious what sort of performance you're seeing on the ingest side with tornado/python. What kind of throughput per server (or systemwide) are you seeing?
When we did the migration over to Storm & Cassandra, we did a 1 time ingestion of old events. This was very fast, peaking around 50k events/sec. Additional synthetic testing showed we could easily beat that. It was actually the old system that bottlenecked, and prevented us from turning the dial way up :)
The ingest side isn't too crazy. We're doing between 50-100 insert-type requests per second per server (although we run multiple instances of Tornado per server so it's actually fewer per Tornado process). The stats on the read side are more impressive. :)