See the wiki page "Abilities", to get an idea of how much each level of Movement Speed Increase gives you.

Yep, roughly 8%, according to our best data. Some people say that it's noticeable. I've never played with both pieces on, so I can't confirm that.

Shock Resistance. Shock is hands down the most deadly status to a Knight.

More so than curse.

good fight Black Kat Cowl.

After reconsidering the discussion in this topic regarding movement speeds of TF2 classes, I think the mercurial speed boost amount (assuming it scales linearly, at 4% per level) is perfectly fine. What we do need though, is a larger variety of gear sporting movement bonuses. At Maximum!, you would have +24% which would be quite noticeable during gameplay; therefore, any piece that adds more than Low must be balanced by some sort of drawback. It could be something as simple as slightly lower defense and a lack of weapon bonuses and status resists, or on glass cannon builds, it might be a one-off 3* piece like Swiftstrike. Perhaps make a trinket capable of adding MSI: Low, that also gives low negative status resists.

I have played with two pieces of Mercurial, and I can say it is quite noticeable after a few hours of play. The 8% boost causes your movements to become much smoother, and when you try to dodge after going back to the base movement speed, you will notice that your knight feels sluggish and slow to respond. To be sure, it is a subtle effect- but it is noticeable, and others have reported the same.

The other way you can tell that 8% MSI makes a difference is when charging a bomb. If you have ever tried kiting an arena while using a haze bomb, you will understand how it feels. While charging, enemies are slightly, but noticeably, harder to outrun. Wearing Mercurial negates that entirely; you can kite like you usually do when not charging.