All right, I'm too lazy to do in depth math, but here's my conclusion, for those Warlocks who care. I'm testing to see how many grenades I get with and without Heart of the Praxic Fire during the duration of the Sunsinger Super. Also, it's late, and I suck at math, as I'm just quickly rushing this test. I might be wrong with my results.
I AM NOT using the perk that gives me two grenades.
Radiance + 334 Discipline (100% CD) + Heart of The Praxic Fire = 8 3/4 Grenades
Radiance + 276 Discipline (100% CD) + NO Heart of Praxic Fire = 7 1/4 Grenades
CONCLUSION: At least with max Discipline, the difference between the cooldowns is worth 2 grenades. However, about 1 grenades worth of that difference is from the last seconds of Radiance, where the cooldown was refreshing the grenade, but didn't finish.
TL;DR: You're only missing out on about one grenade.