I have talked with our EE he has said that the hashrate of 2.2ghs is because of the bitfury chip. He says that this is the average that people will be getting. If you are lucky you can get units that perform higher or if you aren't lucky you could potentially get some worse units it all depends on the bitfury chip. It seems when we build prototypes that we where lucky with the chips that we received.
For fairness, the burden of this 'bad luck' should be carried mainly by the seller, not by the consumers. The specified claimed performance was "Spec: Rated speed 2.6Gh/s. +/- 10%" (
https://bitcointalk.org/index.php?topic=299295.0. You probably need to consider some sort of compensation, for example in coupons on future sales, rather than 'passing the buck' and blaming luck.