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.
A5-A7 Memory Intensive Instances (Standard Tier only)
A5 (14 GB RAM, 2 Cores, 25 cents/hour)
A6 (2x A5)
A7 (2x A6)
These three instances maxed around 700Mbps.
A8-A9 CPU Intensive instances (Standard Tier Only)
A8 (56GB RAM, 8 Cores, $1.97/hr)
A9 (2x A8)
These two instance sizes maxed out at 1.3-1.5Gbps, despite having a 10Gbps interface. These have some pretty good horsepower for transcoding, though.
Naturally, your mileage may vary, but this should give you an idea what to expect from an Azure server when planning capacity.