The limit for a consumer product is the household mains ring.
There is a different limit to the performance. Of course, you can always have multiple chips or multiple devices and get very high hashrates, but the more interesting question is what performance per chip and per watt we will get when the SHA-256 ASICs catch up to the CPUs and slow down the increase in speed. The ASICs went from 330MH/s/chip to 400GH/s/chip (x1200 increase) or whatever the fastest chip currently is in the span of a bit more than a year. I don't think we are going to get 480PH/s chips next year.
As this race reaches the capabilities of current technology, the latest ASICs should be able to earn a profit for a longer time than current devices do because difficulty won't be increasing as fast.
Apparently, 14nm is v. expensive and Kncminer plan to release this Neptune as there final product - due to the limitation on power apparently. I don't know much about processors but surely it's reached it's max nearly. The CPU's for example are gone multicore (of which seemingly will make little difference to SHA256 mining) - what other devices use ASIC's where there is a lot of money involved?
Please bear in mind that my opinions expressed in this thread are v. uneducated and just based on general assumptions about the technology.
Aren't CPU's the same as ASIC's apart from the fact that they just have different uses where the CPU would benefit from newer technology based on market size? What I'm saying is ASIC's are just as much a processing unit as a "CPU"....I'm probably being v. vague here but I've had one or two drinks. It's 11 at night here in W. Europe....