No. Cage length has nothing to do with capacity - the largest cog a derailleur can handle. Cage length only affects how much chain slack there can be in the drive train.
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100% false.
Cage length determines the capacity of a derailleur to take up chain slack
http://en.wikipedia.org/wiki/Derailleur_gears
Changing to a wide cluster > 27 *usually* exceeds most short cage RD's capacity.