I was able to do some speed testing recently with Wowza on Windows Azure. Here's what I found out. One of the nice things about the Manager in V4 is that when monitoring the server during a load test, it's very easy to see where the network maxes out even as connections continue to ramp up. You can set the load test tool to a really high number of connections, feed it a 1Mbps CBR stream, and go to town.
Performance was measured using the Wowza RTMP Load Tool running on a non-virtualized Windows-based WSE4 server at Equinix Virginia, with a 10Gbps connection to the internet (Wowza has delivered over 4Gbps on this machine). Wowza 4 configuration on Azure VMs is out of the box, no special tuning. Test stream encoded with Wirecast. Test is designed to replicate expected performance from internet-connected clients.
A0: Not recommended
A1-A4 General Purpose Instances (Basic or Standard tier, no noticeable difference)
Pricing shown is for Linux Standard tier, Basic Tier is 25% less, Windows instances are 50% higher
A1 (1.75GB RAM, 1 Core, 6 cents/hour)
A2 (2x A1)
A3 (2x A2)
A4 (2x A3)
All four of these instance sizes maxed out around 500-550Mbps.