nitto bar sizing explanation(s)

does anyone understand the significance of the ‘AA’ suffix on nitto bars?

Aluminium Alloy. (eg B123AA)

As opposed to the carbon steel (eg B123) and chrome-molybdenum (eg B123 Cr-Mo) steel models, both of which are chrome plated.

seems straight-forward enough, is there a (significant) difference in performance/ durability?

the AAs are softer (ie less stiff), even the heat-treated ones, but weigh about half (or less) of the equivalent steel bar. A BIT less stiffness probably isn’t an issue unless you’re a gorilla sprinter - a bit of flex adds to comfort for street/JRA.