Looking like the 20mm Sigma will not be coma free, in fact it appears to be about as bad as the 24mm 1.4. This is link to a blog in Poland, that has some good corner crops.
F1.4 is terrible, but by F2.0 it might be useable.
Take a look and pass on your thoughts.
http://www.optyczne.pl/376.7-Test_obiektywu-Sigma_A_20_mm_f_1.4_DG_HSM_Koma__astygmatyzm_i_bokeh.html
Paul C
just fyi, I found an English version of the above article:
http://www.lenstip.com/457.7-Lens_review-Sigma_A_20_mm_f_1.4_DG_HSM_Coma__astigmatism_and_bokeh.html