Out of all the options you have listed the Siggy 70~200 OS FLD outshines all of the others.
Originally posted by Talos Obviously OS on the $1400 Sigma isnt really needed with the K5's inbuilt OS
The OS provided by the simga lens is better on the long end by a stop or two than the in camera SR - also, with the OS of the lens, the VF shows the frame stabilized which allows for more precise framing.
Originally posted by Talos but is its image or build quality really 2.5-3x times better than the $550 version?
Yes - the optical formula/construction with the OS version is completely redone, resolution is pretty incredible.
Originally posted by Talos Also how does the Tamron compare to the Sigma's?
No, does not compete with the OS version. It competes with the others but the Sigma still has the edge with HSM as opposed to the screw drive AF of the Tamron. Also, the Sigma's you can use in MF mode and still have CIF available, the Tamron breaks the use of CIF with MF.
Originally posted by Talos I guess my other option is the Pentax DA* 60-250/4 for a touch extra length (and IQ?) but not sure how it will handle indoor sports events with its slow /4? Can pick on of these up for ~$1500 locally atm so comparable to the OS Sigma in price.
I will let others respond to the 60~250 option. I will note that the Sigma produces excellent images wide open, at 3.2 it gets really sharp and at f/4 it is near mind blowing.
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EDIT: Here are a couple U8 soccer shots made with the OS FLD sigma shot wide open base converted to jpeg and sized for viewing - the FLD elements of the sigma virtually eliminate PF in high contrast shots which is one of the reasons for great wide open performance.
[not great shots, just posted for examples]