I have a few Zotac Amp Extreme GPUs in this test rig, they're factory OC'd. With as little as +100 core, they'll crash. So, unfortunately I can't reliably test core OC
Zotac always had bad reputation for overvolting their poor quality chips to hell and having poor cooling at the same time to the point they're hot like fire. Gigabyte like my 1066 G1 Gaming is overvolted to hell as well but that at least comes with Samsung memory and decent cooling and OC is usually 2000-2100+ depending on the power limit set