My take on the issue
If you run ATI: Use BMS extraction for MFDs and run -Windows mode -full screen or with border has no consequence - even to alt tabbing (instruments can still be used with MFDE, they have much less of an impact than MFDs)
If you run Nvidia: use MFDE exlusively with the refresh tips Darkman provided earlier in this topic. Do not extract MFDs from BMS, it’s too costly FPS wise.
SLI or crossfire don’t do any difference
When you extract displays there are a few basic rules to follow
1. never have one Windows scattered over 2 displays, check your borders. Having Windows rendering on multiple displays has a FPS impact
2. make sure you place secondary screens above main screen to avoid any possibility of display position shifting while switching screen mode or resolution