matt hooks
The problem with ice usually has very little to do with form drag due to an uneven surface, and everything to do with parasite drag caused by an inefficient aerofoil shape.
I'm not sure I'd agree with that. Ice's main effect comes through roughness rather than the shape changes - even the rather extreme double horn shapes used in certification don't do as much damage as you'd expect, but a layer of roughness - similar to sandpaper on the wing - can be horrendous on the leading edge. Because it's very much scale dependent larger aircraft can to some extent get away with roughness (though they shouldn't, of course, try on that basis) but smaller aerofoils can be seriously affected.
The effect is not just in drag but in significant changes to the stall behaviour in some cases, which can in the extreme turn a relatively docile, certifiable aircraft into an uncontrollable beast at much lower angles of attack (and hence higher speeds) than you may expect. This is really a flow breakdown mechanism - it's as if some evil genie snuck up and glued a bunch of random stall strips on your leading edge, in effect. Not something you want to find out about at 50ft AGL or thereabouts ....