It will be hard to beat the KEG because it's in its own league, but it is not impossible if sigma delivers:
1) 1.2A OR 1.4A OS (even better with 1.2A OS).
2) smoother bokeh (the current 85 1.4, 50A & 35A bokeh are behind the canon L's).
3) faster AF then the L (improving AF accuracy too).
4) pricing under 1500.
But I highly doubt sigma will meet all of the above. Maybe maybe 2 of those 4.