Changing a crankset gets you some changes. The weight differences will be minor, and might not be lighter in the end. There might be a need for a different BB, spindle, bearing spec. You could choose a arm length that is better for your style/size if not already had. If you change the number of rings then assume you'll be changing the ft der too. As far as "energy transfer" don't waste too much effort thinking about it. There won't be any difference that you'll ever be able to measure. I doubt your current crankset is getting warm from the energy that you apply to it but isn't getting to the chain
Remember that if there's a "loss" of energy it has to show up elsewhere to obey the laws of physics.
But the real reason to change out a crankset (other then arm length) is gearing. not just the possibility of different ratios but also of shifting performance. I still say that Shimano has the best shifting ootooth profiles, pins and ramps. This goes for the cassette too. At work we have fewer customer complaints about their Shimano toothed bikes then the other brands. But one could just install Shimano rings on their current crankset too. Andy.