Hold, Stun, Confuse, Immobilize, Sleep and Fear enhancements all increase the duration of the effect.
Enhancements are strengthening the power's effects. Archvillains have knockback and repel protection, so that portion of Hurricane is ignored. Archvillains also have innate debuff resistance (87% at lvl 54), and the purple patch makes your debuff only 90% effective (if you're level shifted to 53 fighting a 54 AV). So, you can have Hurricane's tohit debuff enhanced to the max (from -30% base to -71% on my /storm controller) but AV resistance and the purple patch reduces its effectiveness to -8.3% (-71% x 13% x 90%). If enhancements weren't strengthening the power's effects, you'd only have -3.5%. I don't believe AVs resist range debuffs, so that's only impacted by the purple patch and you should be debuffing range by -54% (-60% x 90%).