Originally Posted by timcupery

Yes and no. You're right that the cage length itself doesn't matter; but the "mountain-bike" design, where the derailer is designed to track along a cassette of up to 34t, does matter because it means that the jockey wheel is further away from the cogs as you get into the larger cogs.
But a road-marketed long-cage ("triple") rear derailer shouldn't shift worse than a road-marketed short-cage ("double") rear derailer.