I will be replacing the 28-80 L with the 24-105 L when I get the 6D body + lens kit.
That contradicts what you've said before - if you *really* maxed out the sharpness of your lenses and am not just told so, get a 24-70 mk2 which has prime sharpness but has a slight onion bokeh & lacks the max. large open aperture of fast primes.
Sine you have one prime you should also know if zooming with your feet is something that suits you or if you want/need zooms for flexibility or convenience, a uber-sharp shot on a prime isn't helping if it only fills 2/3 of the frame.
Please post the shot(s) that was criticized in full res & full exif somewhere & we can have a look where/if spending a lot of $$$ would really make a difference in image quality.