Well I went to 344.48 after I heard about all this DSR business and it made changes in the lighting and the performance dropped. That's why I asked if you updated your drivers. I went all the way back to 310 and got my performance back.
As for Inspector settings I use the following: (and this is on a 7 year old XP CPU)
Antialiasing
16xS [Combined: 2x2 SS + 4x MS]
Transparency Supersampling
2x Supersampling
Anisotropic 16X
texture filtering Negative LOD bias clamp
texture filtering high quality
Power management mode: prefer maximum performance
And the biggest performance boost was:
Vertical sync 1/2 refresh rate, then set your frame lock in the sim at 30. You don't need to see 60+ if your frames are gonna' tank.
My old rig runs smooth as glass with this and I can even get FSX with Orbx scenery to run smooth at dense scenery settings.