I just tried this on my GTX980, and while the 3 FPS hit is no big deal, it looks like every few frames the sharpening can't keep up and you get an unsharpened frame thrown in. The net effect is that the screen looks like it's micro-stuttering as the rendering pulses between modes.

The recent addition of AMD's Contrast Adaptive Sharpen (CAS) to Reshade (Here Is The FX Code) is probably going to be a better option for me. It has a bigger FPS impact, but it looks great and runs smoothly.