You may not be able to fit that MTB crank in your Sirrus (is that the bike you're considering upgrading?)

The Sirrus should have a 68 mm wide bottom bracket, but the XT M8000 should be designed around (and include) a 73 mm bottom bracket.

The front derailleur wouldn't have the correct reach from your seat tube to the crank, either. Again, because it's designed around a wider bottom bracket.

I'm not sure if you could re-space the XT M8000 crank (with a shorter spindle) and fit it in a road bottom bracket. Probably not.


Regarding speed, you would lose a ton of top speed because your large chain ring would go from 50t to 38t.


If you want lower gears, you could:
* replace your current rear derailleur with a 9-speed Shimano MTB RD (XT-M772 is a favorite of mine), which would index perfectly with your 10-speed shifters, but allow you to fit an 11-36 cassette, giving you a granny gear lower than 1:1.
* replace your "compact" crankset with a "subcompact" crankset, like a 46/30. The IRD Defiant is an example. That would lower your top gear a little but lower your granny gear significantly. This would probably require a different bottom bracket; most available subcompact cranks use a square taper spindle.