My impression on the efficiency is that they are more efficient but they are slightly heavier internally so there is a tiny bit more loss right there. Under load however the older alternator will have much more drag and heat vs the newer ones and the hotter it gets the worse its output which means it has to work even harder.
Ive been aware of high output alternators way back when I did a big stereo in my lt1 car where I had dimming lights and an ewp AND an underdrive pulley. I didn't know that they only produce the amount of amperage required by the accessories or I would have probably tried this a decade ago. With a higher output alternator my fans would run faster, lights would stay brighter and my waterpump would be spinning faster. I think the benefits outweigh straining the electrical system. Its my belief that with a good high output alternator and my upgraded wiring and grounds the system will be much happier and be more reliable long term. WIth 16 injectors and lots of electronics and wallet denting ignition module failures I felt the upgrade was worth it to make sure there was enough juice for everything to function as required.
This is a neat video I liked. Shows that with fans on, headlights on the difference a high output alt can make. The amp theyre comparing to does put out 170 amps which is a ton more than our stocker.
https://www.youtube.com/watch?v=S83GUurX-no
nice video showing internals. I believe the gm unit there is much beefier than ours.
https://www.youtube.com/watch?v=LMB2Rrpz9yg