Under Stall Speeds, the AFM Supplement S1 states:
Ice accumulation on the airframe may result in a 20 KIAS increase in stall speed. Either buffet or aural stall warning should be treated as an imminent stall.
The TSB report on the 2012 icing / loading loss of the Gogal C208B may well be worth a read.
http://www.tsb.gc.ca/eng/rapports-re...4/a12c0154.pdf