"Test all things; hold fast to what is good" - 1Thessalonians 5:21
Non-drive or left crankarms are both basically interchangeable between road and mountain.
Differences in spindle wall thicknesses between say a 6600 triple and XTR are different. The XTR is thinner. I assume the Dura Ace is also equally thinner/lighter than the equivalent Ultegra.
But I also discovered that the XTR arm bows out more from the spindle to the pedal, therefore, the pedal is more outboard/farther from the longitudinal/fore-aft center line of the BB!
So, there I was thinkin I could slap a lighter XTR arm instead of the existing 105...I could but I'd lose significant pedaling clearance so I kept the 105 on bc it wasnt worth it...oh well, I guess I gotta get a Dura Ace arm then!