Double is probably way too much anyways. Grover's does not parallelize very well. So you would have to do each and every one of those 2^64 operations serially o

Double is probably way too much anyways. Grover's does not parallelize very well. So you would have to do each and every one of those 2^64 operations serially on your quantum computer to get the full benefit of Grover's when cracking a 128 bit key. It is entirely possible that there is more than enough margin in 128 bit keys to prevent a successful Grover's based attack.