I apologize if I misinterpreted.
But even here, I see a better performer in BSD as opposed to Linux.
The firewall I currently use is OpenBSD, and I have not had any issues with it.
BSD - Linux sour grapes aside, the OpenBSD guys do have a point. Linux is messy!
My firewall PC is an old PackardBell :
CyrixM processor 333Mhz with 128Mb RAM.
The only problems I get are with Linux and it's endurance and maintenance.
Particularly installations that use ISOLinux
( Look for older BIOS issues )
This never occurred in OpenBSD.
Even on my own builds, it's messy.
There's over reliance on the parent distribution when I compile. And I can't compile an app on one OS and run it on another. It's all nonstandard code. (e.g. RedHat / SUSE with the one possible exception of a few tricks. But all that trouble for a firewall? )
Back to Windows :
Your note on Windows performance tells me right away, the system is not optimized. A server with that much horsepower should do MUCH better than what you described.
Easy to use means easy to use.
It doesn't mean insecure.
On Windows just as in any other OS, you can still do a better job if the intricacies of it are understood. The service tweaking in *NIX OS's must be done in Windows too.
Your comment on wishing to remove the Windows GUI tells me you haven't done all that you could to optimize the system. And possibly denotes a lack of experience with Windows.
And the bottom line was "specific tasks". As in ease of use as well as security and performance. For which Windows ( with adequate measures ) does very well.
I'm sorry for the bad temper, but this type of thing really bothers me.
Almost like when people complain about GNU apps on newsgroups and say "it doesn't work", "doesn't do it well" or "that will never work". This going back to Avalanche which was thrown out without giving it a chance.