I have a mission that I'm happy with so far that is set up with:

• Chernarus summer using the whole map
• 200 mil placement on each side
• 200 civ placement on each side
• Mil and civ placement ignores small objectives. Performance was also good only filtering tiny objectives.
• Commanders ignore small objectives. Also worked fine filtering tiny.
• Total objectives is 84 per side, worked well at 102 per side as well
• No civilians
• No CQB
• No ambient air, only manually placed air (5 per side) which OPCOM has control over.
• 1000 reinforcements per side
• 1.2km foot and helo spawn range

I host on my PC using TADST and haven't bothered assigning CPU cores yet.

i5-2600k @4.3GHz
SSD
GTX 660Ti
25MB/s Up/Down

So far I've only tested with 4 people at one time. Runs excellent except some lag/desync spikes when people respawn and cause a lot of AI to have to "appear" at base. Other than that things seem ok so far.
The biggest issue for me is that OPCOM freezes after a couple hours.

With this setup my server runs 45-50 FPSall the time except during initial loading and the respawn thing. Obviously the respawn only hurts when nobody else was at base to have those units already spawned in. Fast rides in helos don't effect performance much.