I have the Sigma 35A and it's incredible. I've been eyeing the 50A, but man, I really want lighter, smaller primes.
Then again, maybe I should just buy a small mirrorless system as an alternative for when size/weight matter.
A problem with small, mirrorless systems is that if you want AF and high image quality you can pay quite a lot for the - often considerable - size/weight advantage. The FF Sony a7 series provides the best image quality, but their two primes, which seem weightless and tiny compared to the Sigma 50A, cost (barring sales etc.) $800 (35mm 2.
and $1000 (55 1.
; superb lenses, especially the 55mm, but for that price you might expect something faster. M43 offers a wide range of small, good-to-excellent light primes, but as wsheldon points out the native options aren't cheap (all but three cost >$500) and, aside from a couple of cheap but not-really-fast-enough-for-m43 Sigmas, the third party alternatives are more expensive still (and typically don't provide AF). Plus, the best EVF on any m43 body resides in the OM-D E-M1, which is almost as big as a small dslr and costs not that much less than an a7II and more than an a7. So....
On the other hand, if you don't need AF, you can find amazing bargains and often superb performance with old MF lenses (you could buy ten or more excellent 50-55mm 1.8-1.4 lenses for the price of one Sigma A); and mf is far easier with mirrorless cameras (thanks to in-viewfinder focus peaking and magnification) and ergonomically much more pleasant with lenses designed for mf. But thanks to the crop factor this route isn't much use on m43....
As for the original question, yes, such upgrades would be nice, though I tend to think the flaws of the current EF 50mm 1.4 and 85mm 1.8 are exaggerated. There's obviously some variance in quality control re the 50mm (AF was pretty bad on my first copy), but my main complaint with that lens is that it's unpleasant and tricky to use for mf (because the focus ring is sloppy). As for the 85mm, yes, purple fringing can be annoying, but in many situations I don't see it at all even at 1.8 (it's worse on the 85mm L). Is the much-praised Nikon G significantly better in that regard (if at all)? But if Canon made a new 85mm 1.8 with IS and no (or much less) purple fringing I would probably buy one.