Hi Pat, I have also found the Kenko MC7 DG 2x to be a good TC. Here are a few bird shots with the DA*300 + TC
https://www.pentaxforums.com/forums/post-your-photos/26171-few-birds-da-300-a.html
I also did some test crops of images including the 2x TC which you can find here
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/26491-da-300-s...-some-tcs.html
The 2x requires an aperture of F4 or brighter to get AF, but I can't get AF with the DA*300
even with its F4 aperture. I have to use manual focus.
I agree that left to its own devices metering seems to overexpose, but I generally find -0.5 or -1.0 is sufficient compensation. This may vary perhaps with the lens/TC combination. I suspect in my case that I've been using AV mode a lot combined with spot metering and just with a slight movement of the camera the exposure will change and overexpose the main subject. I would be better using the exposure lock or using manual metering to get full control of exposure.
Paul