Canon 300mm f2.8 L IS + 2x Teleconverter – Real world usage how good is it ?

Sometimes I get in the mood to do big technical tests on gear and lenses. I have a few of those in the works right now but for this one I’m going to keep it simple. Teleconverters have a fairly bad reputation and my first foray into their use was with my original 70-200 2.8 L IS. I picked up a 1.4x II thinking that it might be useful for getting me near the 300mm mark on the cheap. Long story short, I tried it a couple of times and then swore never to put a teleconverter anywhere near my