Don't confuse a road "long" cage derailler with a MTB derailler that can handle bigger cogs. Road deraillers are generally spec'd up to a 28 tooth rear cog regardless of cage length while MTB deraillers can handle up to 34 (sometimes 36) again regardless of cage length. Cage length only determines how much chain slack the derailler can handle (tooth differential between smallest and biggest front chainring plus same for rear cassette cogs).