Not sure how reliable all the Reddit posters are, but apparently the best you can get is a ~30 second cooldown on grenades/melee, and a 4 minute cooldown on your super. That almost makes Intellect seem like kind of a waste. I thought that maxing Intellect meant it cut your cooldown time in half.
http://www.reddit.com/r/DestinyTheGame/comments/36mkuf/sga_choosing_armor_to_upgrade_to_lvl_34/
With all the new gear and old gear to choose from
before you upgrade your armor:
A. Think about your Super(Intellect), Grenade(Discipline), and Melee(Strength) cooldown boosts.
Every 3 points of INT = 1 second faster SUPER cooldown.
Every 8 points of DIS = 1 second faster GRENADE cooldown.
Every 7 points of STR = 1 second faster MELEE cooldown.
271 is the max stat you can hit for each.
No Stats - 0
INT: 5:30
DIS: 1:00
STR: 1:09
Maxed Stats - 271
INT: 3:55
DIS: 0:27
STR: 0:31
*Note: You cannot max all three. You can get about 70% across all of them or max 2 and leave the 3rd.