In reply to ow arm:
Remember you will lose 2 stops with a 2x teleconverter, so your 105mm f/2.8 becomes 210mm f/5.6
I'd expect a cheap 70-300 f4/5.6 zoom to give comparable or better performance at 210mm than a 105mm plus cheap 2x teleconverter. Plus it will give you AF, more versatility and a faster max aperture for most of the zoom range.
You may find you need AF a lot more for wildlife than for macro shots.