"... the Sigma 85mm f/1.4 EX DG HSM is noticeably sharper." (than the 135L)
This just shows how meaningless your new metric is. Your own measurements do not show any noticeable difference. There is almost none here: http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=108&Camera=453&Sample=0&FLI=0&API=0&LensComp=756&CameraComp=453&SampleComp=0&FLIComp=0&APIComp=2
Also, comparing 85 and 135mm can be tricky. For the same background blur, you need f/1.4 at 85mm vs. f/2 at 135mm (same physical aperture). The 135L is then really sharper, much less PF/LoCA.
BTW, nobody buys the 135L for sharpness - not that it is not sharp. It has gorgeous bokeh. Your "second hand" recommendation is really stupid.