I think you need a Hynix card with mining straps running at 2250 and you can go over 1k H/s.
A warning on cheap Hynix cards however, don't go there. You'll end up in RMA hell or forced to run at stock speeds.
A few years ago amd released the R9 280x which is a 7970 with overclocked core and ram speeds in the card bios...
I love to buy these because they mine almost as much as a 380x or 470, as long as you do not factor the electricity cost.
We miners love to overclock, but overclocking the R9 280x - 7970 to the limit also is a don't go there.
The 280x were so often poorly made with cheap chinese knockoff of capacitors (all gigabyte with 3 fans,
some msi and xfx) and bad heatsinks that keep the heat on the ram or voltage regulators (diamond, msi, xfx)
These cards when overclocked at 1011 mhz use 1.2v instead of 1.05v, often locked in the bios;
the capacitors and voltage regulators overheat easily if the fans are under 80%-100%
Think about the gains vs loss of overclocking too much... after all it's your money.