Helios45: if you cant make it work with overdrive tool, then do it the fixed way.
bios mod, hardcode some lower core voltage on the core speed 950 -1200 lines. say....start with 950mv then with your OC tool in the OS try core speed set to 1150 and see how it goes.
I see some weird values for Core voltage in Polaris for frequencies higher than 300mhz, how am I supposed to lower these ?

Those weird values mean you have dynamic voltages. They are reference pointers.
Set all the reference pointers to the lowest value (65282). Keep the lowest state (750) as-is.