So the cap on attack speed appears to be 200% according to the stat page in game, how do temporary buffs like Ghoul, Prismatic Rage or Windshear interact with this? Can they push you above the cap, or do they have no effect past the cap?
What about skills like Pneumatic Burst that aren’t really procs but give additional speed?
Your speed limit is 200%. Past that have no effect, only difference is if you have -attack speed mutator.
Pneumatic burst total speed(including attack speed) is passive buff that applies all the time Pneumatic burst is on.
A good way to think of these percentage-based-stats with a maximum limit is to view them the same way you do with “resists”.
Thus, as @nery explained, overcapped attack speed will function as a buffer against slow debuffs.
How much one needs to overcap attack speed depends largely on how high your slow res is.