I'm curious whether you are using the Tamy at the same F stop as the Pentax. Just because a lens is capable of shooting at f2.8 doesn't mean that its got the adequate DOF for a group at that f stop.
You probably have already thought of the above, in which case i recommend that you do some focus checking of the Tamy lens when you get more time. perhaps its FF or BF for you.
Another question, are you doing these group photos handheld or on a tripod. What shutter speed, iso and aperture would you typically use for a group shot? What camera?
I don't have the Pentax zoom you are talking about, but i do have the Tamy 28-75 and i find it very sharp. Maybe you could have it looked at by Tamron, something doesn't sound right?
Just brain storming here, no offense meant,
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Phil B
K20D, K10D