http://leagueoflegends.wikia.com/wiki/Armor_penetration

Team fights will all be dependent on how well you can set up your ultimates. Black Cleaver + Impure Shots passive with Bullet Time will apply stacks to all enemies that you catch in your ultimate. Gragas' Body Slam to Explosive Cask will help draw in enemies who are now fully stacked with Miss Fortune's passives to actually be followed up on for kills. Because the channel time on Bullet Time keeps Miss Fortune stationary for 2 seconds, then if it were not for Gragas and his Explosive Cask that would have given your enemies time to run away. After those ultimates have been set up your team then can start to make picks with your slows (e.g. Wither) and Nasus' Spirit Fire for more armor reduction. Since this team applies great armor reduction, that makes it good for your midlane to be an AD assassin (i.e. Zed great for picking off all the enemies that got caught in your ultimates), or Yasuo because his Last Breath synergizes with Explosive Cask. Lastly, pick an AP jungler (like Amumu, Evelynn, or Elise, so you can at least have one other AP champion other than Gragas.

You already get movement speed from your passive as well as a 30% slow on your frozen mallet and more slows from your E. Why the **** would you need boots at this point?