Tahiti with the HD7970 GHZ edition Re-launched to be competitive with the GTX 680 and separate from GTX 670 segment.... and the same card rebranded as R9 280X as to cheap out cost they changed a lot of VRM making unable the voltage control and VRM monitorization.. also blocked voltage control implementing that shit called PowerPlay that only work good in the R9 285. all of those things together made the GHZ Edition have a poor OC headroom. averaging 1100mhz up to 1200mhz..
Got it, so 30% overclocks from original silicon = bad.
While GM200/GM204 are seen as amazing overclockers for their ~15-20% overclocks.