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There is a common belief that 'L' lenses are always superior optically to non-'L' lenses. However, I don't think this was ever really the intent of the 'L' lens program at Canon.
Back in the FD era, the 'L' lens designation was simply given to any Canon lenses that incorporated certain exotic optical characteristics, such as the fluorite or (ground) aspheric lens elements. Exotic optical formulations were not needed for "normal" lenses, only for high speed lenses within their respective focal lengths.
All FD lenses of the time were built to the same high quality standards of construction--there was nothing mechanical that separated an 'L' lens from any other lens in the FD program. Likewise, it was expected that a "normal" speed non-'L' prime would deliver the highest quality optical results given its focal length and maximum aperture.
With the EF lens program, things changed, but mostly on the mechanical front. "Normal" lenses were built using plastic construction and intended for amateur use. The 'L' designation was still reserved to lenses with exotic optical designs (i.e., 'fast' lenses), but these lenses were assumed to be favored by professionals, and thus built to higher mechanical standards. Optically however, Canon's EF prime lenses (at least) were still designed to deliver excellent results regardless of 'L' designation; they just didn't need exotic optics to do so. (It may be that the 'L' lens design has even less barrel distortion or vignetting, but that's because those lens characteristics are presumed more important to professional users.)
All this is a prelude to say that, outside of consumer zoom lenses which are obviously designed to a price point, I don't believe ultimate lens sharpness is ever NOT a goal for Canon's designers, 'L' series prime or not. That modern 'L' primes might be slightly sharper than older non-'L' primes has more to do with the ages of their design than the red ring around the lens barrel.
If I am right, then I think there is no reason that a modern non-'L' prime such as the EF 35mm f/2 IS should not deliver results as good as any 'L' lens when stopped down to some common middle aperture. Color, contrast and sharpness should all be of the highest quality Canon can achieve.