I don't consider myself the best destro lock in the world, but I can't get my Fel Imp to do more than 2.2% of my damage (and less than 1% on AoE-heavy fights like Galakras). On the other hand, non-FnB Incinerate + non-FnB Conflagrate + Fel Flame + Shadowburn is usually 30-50% of my damage (even on Galakras). Theorycraft indicates that GoSac would be a 4.5-7.5% buff for me.
So when people are suggesting that GoSac and GoSup are about equal for destro, are they eking more DPS out of their pet or are they relying on the 4 GoSac-buffed spells less?