First off, Byakuya was using his Bankai when Ichigo swatted away his petals.
There is no concrete evidence to back up the assertion that his petals are faster than a bullet. However, i feel confident in saying it because when Ichigo first learns Bankai, he is already noted as being the fastest shinigami while using Bankai. At that level he is already capable of moving faster than the eye can see (keep in mind that we're seeing him just fine because we're watching the show, but Byakuya admitted himself that he couldn't track Ichigo's movements)... now, couple this with that fact that after Byakuya starts directing his petals by hand, the speed increased 10 fold which by all accounts, is faster than the speed of a bullet.
Now, take hollowfied Ichigo, he is immeasurably faster than a bullet... it's not even close. Even if Dante slows down time, all that does is bring Ichigo down to a level where he is visible. As far as the shotgun blast is concerned, i could say the same thing about Dante not being able to stand against a black getsuga tenshou.