I just bought a 17-50. Focus is consistent, but needed +6 AF Adjust on the K-3. I also own a strong copy of the 16-45mm, but so far I'm not missing it. I am really digging the extra stop for bokeh and for getting away from flash.
DXOMark has tests of the 17-50 (OS version) vs. 17-70 Contemporary on the Nikon D7100. Overall sharpness is 14mp for the 17-50, 10mp for the 17-70. To view how they fare at different focal lengths and apertures, click on MEASUREMENTS > SHARPNESS > FIELD MAP
Sigma 17-50mm F2.8 EX DC OS HSM Nikon on Nikon D7100 versus Sigma 17-70mm F2.8-4 DC MACRO OS HSM C Nikon on Nikon D7100 - Side by side lens comparison - DxOMark
The 17-70 is pretty soft at 50-70mm. With all the pixels the K-3 has to spare, I expect you could crop the 17-50 to 70mm and do at least as well.
The Sigma 17-50 beats the Tamron 17-50 pretty easily. The 16-45 actually beat the Tamron at DXO too.
Last edited by audiobomber; 01-23-2015 at 11:28 PM.