My current bike is fitted with Shimano Ultegra R8000 components. I am looking at replacing my 172.5mm crankset with a 160mm crankset. Does anyone know firsthand if the 12 speed 105 R7100 or Ultegra R8100 cranksets will work with my other Ultegra 11 speed R8000 components? From my research, it appears that it will (but shifting may not be as crisp) but I wanted to see if anyone here knows for sure. Thanks!
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Yes they will. The chainline moved out 1mm but it will work.
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Another quality option to consider is a Rotor crank, which are lighter than Shimano’s boat anchors. Shimano lists the weight with 50/34 chainrings as 754 g, and while not specified, I would assume the weight is for a crank length of 172.5 mm.
Even their lowest-tier Vegast crankset is lighter than a Shimano DuraAce crank (same length, same chain rings). They have a Black Friday sale going on, which means you can get cranksets very cheap. Since all their cranks are modular, you can e. g. replace Rotor’s chain rings with a Shimano chain ring adapter and use Shimano chain rings if you can feel the difference in shift quality. (Years ago when I tried them, I couldn’t tell.) Or if you want to put the crank on a gravel bike, you simply buy a new axle.
Quality-wise their cranks are top notch.
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I did exactly this - replacing the R8000 with R8100 on my 11-speed Ultegra setup in order to get shorter cranks. Shifting seemed the same before/after. I did need to go back in to the shop to re-adjust the front derailleur as it was hitting. Never understood why, but KevinKlaes just beautifully explained 
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Shimano has been replacing their 11 speed recalled cranks with 12 speed cranks mated with offset rings to compensate for the change in chain line. So you if can acquire a post recall 11 speed crank, it’s the best of both worlds.
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