I don't see how that solves this problem. there is a string in memory that gets saved on suspend. that string when read by the CPU has the same properties it had before. if the CPU is using rot-13, the string is still rot-13 and the attacker doesn't need to spend the compute needed to crack rot-13, the CPU will simply do that as normal.
How do you see an attacker obtaining the key from memory if not by some kind of cold boot attack or memory swap? Encryption defeats these attacks. An attacker who can read kernel memory via software is 90% of the way to beating you anyway.