Successfully romance Keith David

I can think of two reasons:

1. By making the number of available clusters comfortably larger than the amount needed to upgrade everything, they can add new superpowers or enhancements to the existing ones via DLC without having to redo the map and figure out where to put additional clusters.

2. It means players can max out their superpowers without having to pixel-hunt the entire map to find out where they missed a cluster. Probably an important consideration given that they're not marked on the main city map and the minimap's range is pretty limited...

This is conjecture on my part, obviously , but I think they make sense.

The Saints Row Wiki lists the vehicles in the game and sorts according to their speeds and categories. This data is extracted from the game's files.

From that source, the fastest vehicles in different categories are (info in parentheses are where/how to get them*):

• Hover-type helicopter/plane hybrids
  • Chrome Void (reward for a Virus Collection target located at Sunset Park, Downtown)
  • F-69 VTOL (start a Co-op Cat and Mouse and store it once you have control)
  • Screaming Eagle (a reskinned/retextured VTOL; will be in your garage if you have the Commander-in-Chief Pack DLC)
  • Void (piloted by Zin if at 3-5 bars of Notoriety)
• Planes
  • AB Destroyer (Wesley Cutter International Airport, New Colvin)
  • Snipes 57 (Wesley Cutter International Airport, New Colvin)
• Helicopter
  • Oppressor (Wesley Cutter International Airport, New Colvin)
• 2-person capacity cars
  • Attrazione
  • Wraith (Shaundi's "Under Pressure" side quest reward)

• 4-person capacity cars

  • Blade
  • Genkimobile (Kinzie’s "Gambit" side-quest reward)
  • Hammer
  • Infuego
  • Pacemaker

• Motorycles / Bikes

  • Angry Tiger (reward for Asha’s "Training Day" side-quest)
  • Kaneda
  • Wireframe X2 Phantom (reward for Virus Collection target located at Bridgeport, Carver Island)
  • X-2 Phantom (Shaundi’s "Under Pressure" side quest reward, or grab one during a Virus Injection diversion where it could be driven by enemies)

If there is no unlock/reward info, then that means that the vehicles are either randomly parked or driven by NPCs. The 'unlockitall' cheat and the Additional Cheats mod (PC only) can also add many of those vehicles to your garage.

^{Source: Personal in-game experience; some info are from the vehicles' respective Saints Row Wiki articles.}