I have shot not with a 28 to 300 but have several 28 to 200, Sigma in Minolta Mount and Tamron IF 28 to 200 in Pentax AF K, of the 2 I find the Tamron to have faster AF, both sharp in the mid range, seem a little soft at 200, have not had time to chart test. From what I have seen in reviews the 28 to 300 are very much in the same ball park as the 28 to 200, perhaps a little soft at the long end. Don't know if either Sigma or Tamron make an IF in that range.