Dumping data here in case I give a **** about it later. 3x Greater Quintessence of Movement Speed gives a 4.5% boost to your MS. Champions with different base movespeeds will be affected like this:
  • 325 -> 339.625 (+14.625)
  • 330 -> 344.85 (+14.85)
  • 335 -> 350.075 (+15.075)
  • 340 -> 355.3 (+15.3) (This makes you faster than <330 MS champions with boots 1)
  • 345 -> 360.525 (+15.525) (This makes you faster than <335 MS champions with boots 1)
  • 350 -> 365.75 (+15.75) (This makes you faster than <340 MS champions with boots 1)
  • 355 -> 370.975 (+15.975) (This makes you faster than <345 MS champions with boots 1 and <325 MS champions with boots 2)

It is worth noting that ranged champions who take MS quints and have a Phantom Dancer will be faster than most melee champions who purchase Trinity Force, despite melee champions having higher base movement speed.

How MS quints affect the flat bonus from boots:
  • 25 -> 26.125 (+1.125)
  • 45 -> 47.025 (+2.025)
  • , boots 2 + 60 -> 62.7 (+2.7)
  • + 75 -> 78.375 (+3.375)
  • 105 -> 109.725 (+4.725)
  • + 120 -> 125.4 (+5.4)

This should also explain why Janna's Tailwind is one of the most overpowered passives in the game. It's like two free MS quints for everyone on your team.