The point of using studded tires--or winter riding in general--is don't make sudden moves! Even studded tires take a moment or two to grab, especially when the surface covering is very loose and/or slippery (deep slush or wet ice). As for slipping when unclipping/pulling out of a PowerGrip, toe clip? I don't find slippage to be a problem.
Jesse...you might be leaning the bike too much out of habit...try not to lean it like that when you pull out of the pedals.