well your cooler might be better than mine
Zalmans are known for looking good, but mediocre on performance/price. But even if slightly better (which I seriously doubt, having used Zalman coolers before), that wouldn't explain such a large difference in temps. Was the 'stock thermal' compound one of those pre-applied deals, or included as a tube/bottle with the cooler? IOW, did you apply the compound before seating the cooler? I prefer to apply the compound myself, putting a small pea-sized amount on the IHS, spreading it out in a thin and even layer with a piece of plastic, then spread an even thinner layer on the bottom of the cooler. The idea is a thin-as-possible layer with no gaps.
I'd also check to make sure your fan speed on the cooler is adequate - mine is around 2700 rpm under load, which is still pretty quiet for a 120mm fan. Also, vcore is 1.02V at stock and idle.