Interesting. I'm a huge fan of Windows Server 2008, evenmore about the R2 edition (windows 7 server), though not specifically for Game Servers.
Though all my game servers run on them. I'm running Hyper-V hosts, with Virtual Machines inside that run the gameservers.
Piece of cake for me to setup a VM with Windows XP and run ArmA inside it, after seeing your results, I think I will, even only for testing
It would be interesting to know what was causing the higher cpu load aswell as the lower FPS on the 2008 installation.
For all other purposes than gaming, I see improved performance compared to Server 2003 for instance.
Nice overclock btw
If you are still able to test with the 2008 Server OS, I would love to see the result after you disable the different tcp/ip enhancements 2008 has enabled by default. (With certain hardware, these features completely mess up network traffic :P):
(Run from Elevated Command prompt)
optionally you could try this one too:
netsh int tcp set global rss=disabled
netsh int tcp set global autotuninglevel=disabled
netsh int ipv4 set global taskoffload=disabled
You can review the default settings if you like with these commands:
netsh int tcp set global chimney=disabled
netsh int tcp show global
netsh int ipv4 show global