You're comparing different operating systems with different drivers, yes. But that's the point... this is what the end user will see. The open source driver is the default/standard on Linux systems, whereas Catalyst is the only option for Windows. It makes sense to compare whether or not Radeon on Linux can compete with Catalyst on Windows, especially when using "legacy" hardware.
I'm really happy with Radeon's progress lately. Still too slow for "real" usage, but it's really getting there. I don't even think of FGLRX anymore.