Welcome to our forum.
Vinny, I found when using my 2x multiplier, it knocks down the light entering the lens. If you compensate for that you will take great pictures. It has little effect on nice bright sunny days.
You are correct. Using a 2x teleconverter will reduce your exposure by 2 full stops of light. Depending on your camera and lens, you may lose auto focus.A 1.4x teleconverter is a MUCH better choice if you must use one. (you only lose one stop of light)I have used a 2x on my 70-400mm 2.8L exactly once. The image degradation quite noticeable.Oh, and IMO, unless you're a pro and nail the white balance and exposure every time, you should be shooting RAW. Skip the JPEG all together.Just my $0.02-et