270X is just a renamed 7870 OC, it has the same Pitcairn core. So on the same clocks, the XFX should be faster. If you really want to OC it, then buy a second-hand cooler with vrm cooling (e.g.: Arctic Twn Turbo, Alpenföhn Peter, Zalman VF etc.) - it costs less than 50$ -, and set the voltage in VBE to the maximum plus set the TDP limit to max (+20%) too. I think that may be enough for an 1200-1250Mhz clockrate.
Some
Google-translate crap, it may help you with setting the ideal TDP limit and voltage.
The only software solution for higher voltage is to modify the voltage table, but VBE doesnt support it yet, so you have to find it out on your own.
One of the maker of VBE7 is active on a non-English speaking forum what I visit recently, so
I can ask him again, whether he will continue the project or not. Darkhmz (who u can find on TPU forum) has been inactive for 10-11 months.
I have no much knowledge about OCing, so I can't give you more useful advices.