Soooooo. With momentum you are getting bonus dmg as you said, 100% + bonus ms% divided by 2.
When i had whirlwind 6M dmg it went to ~8.5M dmg and with momentum and 25% bonus ms it went from 6 to 11M.
You loose about ~23% dps but you gain free affix and +25% move speed. You could for example change MH item drops for 2x extra attack chance or weapon damage and then switch move speed on helm to item drops.