<p>A little thinking off the top of my head on teleconvertors:<br>
The whole point of a teleconvertor is to magnify the image <strong>before</strong> it is stored on a, less than perfect, medium.<br>
If the storage medium (the image sensor) was perfect, and had unlimited number of pixels, no one would need teleconvertors! We could just capture the image with any focal-length, and zoom in to the recorded image later.<br>
In the past with grainy film, zooming into images quickly gave high loss in detail, so using a teleconvertor to magnify the image before it was recorded made great sense.<br>
<strong>I profess that by now, with 20-50 megapixel sensors, this is totally obsolete.</strong><br>
Since teleconvertors intruduce loss of sharpness, contrast and other tings (like camerashake), there is a point on the line from the low resolution sensor to the very high resolution sensor where you no longer get a benefit, but rather the opposite no matter how hard you try.<br>
In my opinion it is always better to <strong>crop</strong> the image from your high resolution sensor with your longest tele-lens first, than use a teleconvertor.<br>
If even a 100% crop gives too little magnification, than be my guest and use a teleconvertor. (Even if a further upsampling actually could give a sharper image still...)<br>
Telconvertors are produced and used by old habit, and should be abandoned. They will soon, trust me :)</p>