When a bard uses the extraordinary ability versatile performance, the bard does not use the picked Perform skill specialty as a substitute for the associated skills; instead, the bard "can use his bonus in that [Perform] skill [specialty] in place of his bonus in associated skills."
That is, the bard does not, for example, use his Perform (comedy) skill instead of his Bluff or Intimidate skill but can substitute—wholly and completely—his Perform (comedy) skill modifier for his Bluff or Intimidate skill modifier when he makes a Bluff or Intimidate skill check.
This makes it legit for a such a bard to use, for example, the bard archetype street performer's special ability quick change to take 20 on a Bluff skill check yet see that Bluff skill check use the bard's Perform (comedy) skill modifier rather than the bard's normal Bluff skill modifier.
The bard archetype street performer's natural ability streetwise grants the bard, in addition to other bonuses, "a bonus equal to half his level on Bluff… checks made to influence crowds." Because this is a bonus to Bluff skill checks, this bonus is replaced by the bard's Perform (comedy) modifier if the bard uses versatile performance to use his Perform (comedy) skill modifier in place of his Bluff skill modifier.
In short, the bard does not get to pick which of his individual skill bonus and penalties are replaced. Either the bard uses all of his Bluff or Intimidate skill modifier and makes a Bluff or Intimidate skill check or the bard uses all of his Perform (comedy) skill modifier and makes a Bluff or Intimidate skill check.
The bard does not get to use a combination of the two. For example, a bard can't use his Perform (comedy) modifier to make a Bluff check and apply to that skill check a bonus to Bluff checks via the ability streetwise because any and all modifiers to Bluff skill checks are utterly replaced by the bard's Perform (comedy) skill modifier.
Now, if you'll excuse me, I must rid myself of this enormous headache. Thanks, Pathfinder.
