I just did another complete round of testing (240 images) and I am convinced that my 70d does not have an issue even with lower, diffused lighting.
- 70d with 24-70L, and with 50MM 1.4 sig
- at wide open aperture, and stopped down to f4
- with liveview, center point viewfinder, and lower point viewfinder
- low iso, mirror lockup, solid tripod, good lighting, refocus for each shot
- SOOC analysis with no sharpening
- high contrast target, shot at 18', flat plane parallel to sensor plane
- target on flat wall, no objects to distract focus closer or farther
Results: Quite simple. 100% hit rate for liveview, center point VF and lower point VF, no misses in focus, no advantage in LV. As expected, stopped down (f4) did see a small improvement in sharpness from wide open (f2,8) but this was consistent with all three methods of acquiring focus.
Sample images are result of f4, center point viewfinder at full and 100% cropped sizes. I do not have a method to upload more than 2 images at a time but the entire sample set were, within reason, exactly the same as this.
Conclusion: I cannot see any advantage in LV over VF focusing with adequate lighting present with my 24-70L lens. Next I will test low lighting scene under same controlled conditions at 18 feet.