It all depends on the wing section. In the past, I have flown many different sorts of gliders and each reacted to ice differently. The older types with non-laminar sections generally appeared not to be bothered by ice (maybe an increase in 10% or so with a thickish layer of ice) but some newer types (DG500 for instance) would fall out of the air with one or two bugs on the leading edge. And if you made cloud climb through the icing level you might find that you would have to add 50% on to your normal flying speeds to keep the damn thing flying with only just a trace of ice on the leading edge. Therefore only the safe way to fly in icing conditions without ice protection is to use the "Calendar Method" ie. wait untill you get a month with no ice (or, I suppose, change levels).