...a FAI gains you as much difference as available, minus the opportunity cost of FAI's development...
Exactly. So for building FAI to be a good idea we need to expect its benefits to outweigh the opportunity cost (we can spend the remaining time "partying" rather than developing FAI).
For example, one possible effect is a few years of strongly optimized world, which might outweigh all of the moral value of the past human history.
Neat. One way it might work is the FAI running much-faster-than-realtime WBE's so that we gain a huge amount of subjective years of life. This works for any inevitable impending disaster.
http://www.huffingtonpost.com/stephen-hawking/artificial-intelligence_b_5174265.html
Very surprised none has linked to this yet:
TL;DR: AI is a very underfunded existential risk.
Nothing new here, but it's the biggest endorsement the cause has gotten so far. I'm greatly pleased they got Stuart Russell, though not Peter Norvig, who seems to remain lukewarm to the cause. Also too bad this was Huffington vs something more respectable. With some thought I think we could've gotten the list to be more inclusive and found a better publication; still I think this is pretty huge.