The last thing that I see could have an effect is the resistor connected to MODESEL pin (pin 40), but without the datasheet, there is no way we can figure what it's supposed to do.
they could have a few multipliers/dividers for frequency that are related to the value of the resistor.
If I had a board with a single chip on it, it would be easier to try this kind of things.
If the chips were at any time able to hash 300Kh/s each, there must be a way to activate it, and if they claimed 300 Kh/s, it also means that the chips were able of more.
Is there anything written on the chips for the Blizzards? On my Fury the chips are blank. Not even a Zeus label is etched on them...just a nice flat black surface ready for etching. If there was at least SOMETHING on the chips...that could help.