re benchmarks...publishing some is on the TODO list. See https://github.com/zettio/weave/issues/37. Informally, weave is pretty fast but it's not saturating Gbit Ethernet. As you say, capturing with pcap and handling packets in user space carries an appreciable overhead.

We've got some issues filed to look at pcap alternatives and also generally aim to improve performance.

re suitability for NoSQL clustering... depends on where the bottlenecks are; if you want to cluster for HA rather than scale, i.e. there aren't any real bottlenecks, then weave will work well. Same if you want to cluster because of CPU or memory bottlenecks. If, otoh, networking is the bottleneck then adding weave into the mix isn't going to improve matters.