FYI for anyone wanting to buy from Philip I have sent to him upfront and deal went very well...trusted seller for sure.
They can be overclocked (voltmodded) to 7MH, but will need extra care like heatsinks on the power mosfets, and another heatsink on the bottom of the PCB where the power mosfets are located. Secondary side fan to cool the power mosfets is almost mandatory if you want to reach 7MH. I reached 7.2MH on the ones I modded.
Extra cooling of each individual chip may help reducing HW errors too.
You can have a look at my topic for overclocking Gridseed Blade to 7MH.
Basically, overclock = replacing 1 resistor on each PCB and adding heatsinks and a fan.