Man, I miss college. In college I took some thermo classes, which was not related to my course of study, but I found it fascinating nevertheless. Now it's all over my head--wish I could just start over again in college, would be so much fun...
Anyhow. IIRC there is an upper limit to Carnot efficiency. Again, past me, but I don't think we can get much more from reciprocating engines? I'll leave it to people far wiser than me to explain it, using terms that a 5 year old can understand.
On the flip side, hitting 41% means that we're not far from an upper limit to mpg. Nothing will hit 100% so this kinda/sorta says we can never do more than about double the mpg that we are getting today. That's for an engine under continuous usage. A hybrid starts and runs the engine under high load, so when the engine is on, it's at this 41%. It's not 41% at idle, and it's probably not 41% at 100% WOT / max horsepower. By running in a pulse and glide sort of manner, hybrids eek out what there is to be had. It's going to be hard to get more I think. I could be wrong, but it takes a certain amount to overcome rolling losses, drivetrain losses, and air drag.
*
I believe in order to hit 41% efficiency they are running Atkinson cycle. Doing this means the engine gets less volumetric efficiency, which means less horsepower per liter. I suppose that is the downside. After that we can wring our hands about DI and 0W16 and electric water pumps and electrically controlled thermostats and electronically controlled oil pumps--all of which are being used to eek out all there is.