"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!)