People that have used or still use the smaller 5-chip Gridseed Scrypt ASICs and have done a voltage modification to them knwo that these small miners were able to handle a significant performance increase without trouble. The reason for that was the fact they they were originally designed to be used for mining both LTC and BTC, thus they could handle higher voltage and operating infrequence easily if you only used them for Scrypt mining. The case with the ZeusMiners however is not the same, it seems that their miners are already pushed quite high in terms of power usage and operating frequency not leaving you with much headroom for experimenting. Voltage modification of a ZeusMiner Blizzard or GAWMiners Fury is possible and can be done with the replacement of resistors just like on the Gridseed ASICs, however the advantage you can get in terms of hashrate increase is not going to be that attractive so that a lot of people would be willing to do it. There is a topic on the Bitcointalk forum where people are trying to modify their devices and are reporting some useful information and their experience and results, so you might want to check that out if you are interested.