Update: I received the Tamron 28-75 yesterday and took it down to the wedding site (outside wedding) and shot a few. After making many comparisons with my old Tokina 28-70 ATX AF 2.8, I am sending the Tamron back. It is an excellent lens, sharp even wide open, but is only slightly better than the Tokina, and once you stop down, no better at all. The Tokina is all metal and focuses as fast and accurate. No point in having two such similar lenses.
I went to Photozone last night and did a lot of comparisons in this general range....I was surprised at how close all the models were, regardless of brand. I have a Sigma 17-70 HSM OS 2.8-4.0 and I might even be tempted to use it. It is lightning fast and deadly accurate, and renders a little richer color.
It is not as important to have a super fast lens on the K5 as it once was in the past. The added ISO abilities can make a slower lens plenty fast for all except the lowest of light conditions.....I keep forgetting this!
Regards!