A) Hashing an already hashed password does nothing for us. How is an ASIC (that only does hashes) gonna find a twice hashed password?
The point was that if the ASIC operates by hashing twice internally, then you simply hash the hashed password from the DB and then spin through the combinations looking for the twice hash. So, the basic answer here is that, yes, these devices can be used to crack SHA256 passwords that have been either hashed one or twice. And most 8 character passwords will be cracked in a reasonable time.
Did you think that through?
So ASIC goes input -> SHA256 ->SHA 256 and can do a large number per second.
However you only have the stored password which is single hashed. Your solution is to hash it again. Ok so on your slow non-ASIC you are going to single hash the hashed pasword and compare it to the double hash output of the ASIC?
So more work and it is no faster than simply using your non-ASIC to single hash the plain text.
Yes, I thought it through. You only have to hash the hashed password once on your CPU to get the comparator. Then you run the 15 exohashes on the ASIC to find the match.