If you're interested in specific numbers, I've created some graphs at various HP levels to illustrate when the new Fel Armor surpasses the old Fel Armor. Everything can be found at:
http://felfire.com/warlock/fel-armor-remake/
Here's the summary:
At 100,000 HP the new Fel Armor is a HPS increase starting at approximately 13,000 DPS, and a HPS decrease below approximately 13,000 DPS
At 120,000 HP the new Fel Armor is a HPS increase starting at 16,000 DPS, and a HPS decrease below 16,000 DPS
At 140,000 HP the new Fel Armor is a HPS increase starting at approximately 19,000 DPS, and a HPS decrease below approximately 19,000 DPS
At 160,000 HP the new Fel Armor is a HPS increase starting at approximately 21,000 DPS, and a HPS decrease below approximately 21,000 DPS