“How much smarter than a human could a thing be?”—almost infinitely if it consumed all of the known universe
“How about the same question, but using no more energy than a human?” -again the same answer—assuming we assume intelligence to be computable, then no energy is required (http://www.research.ibm.com/journal/rd/176/ibmrd1706G.pdf) if we use reversible computing. Once we have an AI that is smarter than a human then it would soon design something that is smarter but more efficient (energy wise)?
I have to agree with rlsj here—I think we’re at the point where humans can no longer cope with the pace of economic conditions—we already have hyper low latency trading systems making most of the decisions that underly the current economy. Presumably the limit of economic growth will be linked to “global intelligence” - we seem to be at the point where with human intelligence is the limiting factor (currently we seem to be unable to sustain economic growth without killing people and the planet!)