Only if they've developed an ASIC that is orders of magnitude more efficient than what anyone else has - which seems unlikely given the enormous economic incentive to find more efficient hardware.

I'm not sure if you understood my post right. Bitcoin mining relies on ASICs. If the NSA wanted to do a 51% attack, then they'd have to spend a bunch of money building ASICs that are then worthless to them once they're done with their attack.

If Bitcoin mining relied on GPU or CPU computation, then the NSA could just redirect some of their existing GPU/CPU hardware to do a 51% attack, and then go back to using it for something else when they were done. It would be cheaper and easier for them to do.

The only reason to invoke NSA in this context is for their ability to make a 10000x investment, with no concern for this being (directly) economically viable. The question then is, what will they do with control of the blockchain?

Idea: prevent North Korea from spending their coins?