[jatrax]

+ 2x teleconverter

Hardly took a shot worth while keeping.

There is a relationship between these two statements. There are decent teleconverters out there. They are not cheap and are often 'dedicated' ones designed to work with a particular lens or lenses. But IMHO adding a cheap 2x TC is going to deliver images no better, and most likely not as good, as just cropping the original without the TC.

The exception that I have used is the new Pentax HD 1.4x RC, which does seem to work well and deliver images better than a crop.

[RKKS08]

I had this TC, but the original one from Kenko, and additionally the macro converter.
I have heard the combined tele/macro version is IQ wise not as good as the TC only one.

But I replaced the TC by the Pentax Rear Converter-A 2x-S, which is better, SMC, and also smaller.
And the Kenko macro converter was replaced by a cheap offer of a SMC Pentax-M 4/50 macro. That's not 1:1, but even the shortest extension ring cares for that, if really needed.

And my experience from the eighties:
Zoom lenses rarely like TCs, if not specially paired by the manufacturer of both items. No Zoom lens is 100% optically corrected throughout the range, and if the lenses are not paired, remaining lens faults may get worse.