Momentum (3) in arena means move speed *30% . So 100% move speed x30%= 30% increased dmg from bonus move speed such as from Ruby and skills, talents and sets.
7.5% move speed would mean 2.25% more dmg. 10% would be 3% more dmg since its 10% of 30. 24% move speed means it would be 24*30%=7.2% more dmg. However the cap move speed applies to Legend affix but the Skills like Shieldwall can go above since the native cap in PvE is 100%. If the native cap is reduced, it would mean you could get 40% move speed just by Shieldwall alone since it does count.
40% move speed would be 12% more dmg then with Momentum (3) plus shieldwall does reduce DMG by 40%. Can be increased by hero points. Well Momentum can add DMG in PvP but in my opinion, most builds that DMG isn't that huge compared to other sets in PvP. It isn't completely useless though and if you could get the 100% actual move speed (capped 24% legend affix +skills) , then it would be something. Easy ways to have high move speed is shieldwall as well as Knightscharge, coat skill. With summons, you can easily get 50% move and attack speed in arena with Synergy Mythic affix.
As for PvE. Well interesting that Momentum seems to reduce your DMG with no move speed in your case. I never noticed this but I did feel it doesn't increase DMG unless you add bonus MS, since it doesn't count your own movement being 100% default or 1. Well it used to I remembered but now it only includes bonus MS from skills, sets, talents and legend affixes for damage increase.