if your target for 5 years better go for stock clock..
and 70 degree for this time, later it will increase more than 70 degree.. and if 70 degree on idle mode is bad for gpu, if u are gaming with it get 70 degree still ok..

i agree with u.. some part got voltage specification, if the part cannot effort the custom voltage will easily defect..
also increase the voltage will increase the temperature too..
TS also might have to buy water cooling system if want to do hard OC..

i mean the electronics part mount on the gpu board, like diod, transistor, power regulator, i/c and many more have own itself voltage/temp spec.. u will not find the part characteristic unless u work with them, know the circuit and material they used..
usually people doing oc to enter a competition.. its just temporary oc to break the record and u have to adjust everything..
not sure where to find max temp since amd prefer not to show max temperature on the specification..

i will put the speed as 1500mhz if the cooler also cooling the RAM chip..

not cooling defect, 1 of those vram chip unable to effort that speed or defect.. usually vram chip have body mark as part number.. u can check over the internet about the spec of the vram.. maybe different version..

not cooling defect, 1 of those vram chip unable to effort that speed or defect.. usually vram chip have body mark as part number.. u can check over the internet about the spec of the vram.. maybe different version..

still have chance.. except u modified bios..