After reading this post, I bought the Tamron 10-24 from KEH, thinking it would make a nice affordable wide angle for my K-1 II, figuring that I would use it at 15mm and up.
My results were quite different from OP. There was significant vignetting at 15mm, even after vignette correction with the lens profile in Lightroom.
Here is an example image taken with the lens, without lens correction, that shows the vignetting.
Worse, to me, the image is just soft all around. Certainly way softer than the Sigma 10-20 f4-f5.6 that I sold to a forum member because it had a black circle in FF mode. At least the lens was always sharp in APS-C mode. This lens is just blah.
I have taken it upon myself to print test targets to look at things like corner sharpness at various focal lengths and aperture. I don't do things halfways, so I printed 9 targets of 13x19 each, so I could properly test the ultra-wide angle lens. That's a 39 x 57 target. Not perfectly flat, mind you, as they are all taped together (that took a while!). But I checked with other lenses known to be sharp (my D FA 28-105) and it was more than acceptable to test.
I put my camera on tripod at ISO 100. SR disabled. Focus was done in live view, then camera was put into MF mode. Shot with IR remote with 3 second delay.
All the test shots below have had lens correction applied in LR. They look even worse without it in terms of vignetting and distortion.
I tested full-frame mode at 15mm, 20mm and 24mm, at all apertures from wide open to f16.
At 15mm, corner sharpness was horrible regardless of aperture. Here it is at 15mm f16.
At 17.5mm, vignetting isn't quite as bad. Sides are still unsharp. Corners not easy to evaluate due to where they fall.
Here it is wide open at f4.5 .
And at f8 .
At 24mm, sides and corners are still blurry at f4.5 wide open.
Corners and sides improve somewhat by f8, and more at higher apertures, but remain uneven. Top left corner seems sharpest. Here is what it looks like at f16.
I also tested with the Tamron MC4 1.4x PZ-AF teleconverter.
First, I tested at 11.25mm, or 15.75mm with the TC. Wide open, still poor results in corners/edges.
Edge/corner sharpness is still poor at f8. It looks better at f16.
The 15.75mm/f16 image below might almost be a serviceable image in some cases, but the vignetting remains bad, and even 200 setting for vignette correction in Lightroom doesn't fix it all.
I also tested at 12.5mm on the lens which is 17.5mm with the TC. Results are pretty similar to the 11.25/15.75 setting. Vignette still not fully correctable. I'll spare you the images. I didn't test at any other focal length with the TC.
Finally, I tested the lens with the camera in APS-C mode, without TC, ie. the intended usage for the lens as it was designed.
At 10mm, f3.5 (wide open), it's actually not half bad. LR did a good job of completely correcting the vignette.
It gets better at f8, and better yet at f16 as shown below :
I also tested in APS-C mode at 16.88mm and 24mm. I don't have a lot to say about them. The lens is OK wide open and decent stopped down.
Clearly, the Lightroom lens correction works very well when the camera is in APS-C mode. Not so well at all when it's in FF mode. And forget FF mode with the TC. Guess one would need to reprofile the lens for full-frame. But it's not actually worth it.
I think if shooting on full-frame, you really have to use a teleconverter with this lens, and stop it way down. Otherwise, you are simply better off putting the camera in APS-C mode. I'll probably return the lens to KEH, unless someone at Pentaxforums wants it to shoot in APS-C mode, in which case I think it does the job, but that's not the job I bought it for.
I'm not sure if I want to continue to play the game with trying another wide angle APS-C lense on full-frame. The Pentax 15-30 is probably the way to go, but it costs an arm and a leg. I see many Tamron 15-30 for Canon and Nikon in the $600-$800 range on ebay. No used one for Pentax. Brand new is $1300.
Are there any other full-frame ultrawides for Pentax ? The Sigma 12-24 DG comes to mind, but I don't know if it's any good. Lots of them on ebay for $300 - $400, but in Nikon mount. Two for Pentax at nearly $600 in Japan. I don't think I will bite as it's an older design and non EX / HSM version. And too much hassle to return to Japan if not good enough.