8192 bit RSA key is one or two notches above overkill. 2048 is expected to last many more decades, and I haven't seen 4096 outside tin-foil hat environments. I know our national root certification authority uses 4096, for example.

I propose such a key actually decreases security because it is less compatible with existing software, training the users to further ignore warnings or dangerously tinker with their security settings.

Which one is more likely? NSA being able to break 4096 keys, but not 8192, or your user accepting an invalid certificate because "the key is probably just too big", or "it drains too much battery"? Which one will have more impact down the line?

Besides, if you are worried about such sophisticated attackers, your priority should be securing side channels and protecting yourself from rubber-hose cryptanalysis (http://en.wikipedia.org/wiki/Rubber-hose_cryptanalysis).