I'm going to South Africa next year, and don't really want to rent a 300mm lens. I have a 70-200 f4 IS that i'd like to use, and thought a teleconverter would be a smart idea. B*H has a Kenko 1.5x teleconverter for under $90. Assuming that i'm going to be using this in good lighting, how much worse are these than the $290 Canon 1.4x? I assume both take away a stop (f4 to f5.6) ... am i going to see a big difference in IQ between the two units?