Mods saw a decrease of about 3fps, but my mods were just extra SP missions and a custom insignia. I was discouraged when I got the results. I thought for sure I wouldn't be able to run it even on low, but that was not the case. :)
My results:
http://steamcommunity.com/sharedfiles/share/?id=1133612332
My setup:
CPU: i5-4690k @ 4.2Ghz
RAM:16gb [no OC/default speed, I'll check speed and edit this]
GPU: MSI R9 280 [no OC]
MONITOR: 1920 x 1080p
STORAGE: 1T WD Hard drive, 7400 RPM
OS: Windows 7 64bit
I'm using the absolute latest drivers for my GPU, which is Crimson Radeon ReLive 17.8.2, and the default build for Arma 3. I can run SP very well on mixed Very High-Ultra, with some settings shut off because I personally don't like them or need them. Online, on good KOTH servers, I usually get about 40-50 FPS, sometimes even 70 if the server is really good.
Beyond that, I haven't made any changes to parameters beyond setting it to 64bit update.