Not just at 2nl. If someone at higher stakes is 3bet jamming 400bb over 3bb open then they are a fish and you should disregard any 'oh its NL50, people are aggro stuff'. He either has AA or is on ubertilt. Mostly AA as ubertilted guys tend not to have 500bb stacks.
It's rare that you'll be put in this spot and have 400 bb vs a shove. I don't really see this spot happen, it's very rare apart from when these two hands battle it out. Unless you see this guy do anything crazy all the time then maybe we have to fold for this amount. So I guess the question is... at what point stackwise do we start to fold KK pf? Of course it's villain dependent too.