I'm moving this to the DSLR forum, since that seems more appropriate. I suppose I could have moved it to the lens forum, but this seems perhaps a DSLR-specific question.
Anyhow, I'd agree the combination of a completely manual TC (one with no aperture contacts) and lens without an aperture ring is not a good one - you'll have no way to control aperture at all. It will either be wide open or fully closed, depending on whether the TC is pushing the aperture lever or not, I suppose. I'd expect a K-mount TC to push the lever. I suppose if for macro photography you preferred the aperture fully stopped down, you could modify the TRC to do that.
I haven't used the Kenko 2X TC, but given that you lose two stops of light with it, I'd expect that combining it with a 90/2.8 lens would yield a 180/5.6 lens that was in most respects noticeably worse than the DA50-200 or any other consumer zoom lens at that focal length. But you would of course keep the minimum focus of the macro lens, so you'd now have more magnification.
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