I have a 70-200 2.8 and the EF 2x II, and finding a longer lens without spending multiple thousands is hard!
I am VERY satisfied with my 400 5.6 IS with the 2x, and was looking to get something longer, and the 300 F 4 IS looks good but doing some math, i really only benifit when using the 2X on it, over using the 70-200 W 2X..
Lens Math
I know its not a good idea, but really would a 1.4X and a 2X on the 70-200 be That bad? Anyone else do this? Ive put my old 4 element 2x on top of my canon 2X and it really doesnt look awful and thats the worst of the worst so...
Should I spend $1000 on a lens, that weighs 3lbs and will only REALLY benifit with the 2x?

OK, I have a 1.4 TC and a 2.0 TC. I had a 70-200mm f/2.8 (sigma). I had zero issues using the 1.4 on it, but for me the 2.0 made too many compromises. I've since sold the 70-200 for a 135L and I also picked up the 300mm f4 IS L you're questioning.

First the 300mm blows the 70-200mm w/1.4 out of the water. Second it takes TCs better than the 70-200 ever did. I've used it with the 1.4 and really can't see ANY ill effects there, and I've used it with the 2.0 and both stacked with OK results. I still wouldn't use the 2.0 or stack them if I can avoid it though because I can see IQ issues with just the 2.0. Add to that the fact AF is hard with the 2.0 and impossible stacked it's not worth it to me.

For me the 1.4TC is an awesome piece of equipment. But once you need more reach than that, a lens suited for your needs will get you better results than a 2.0 TC, but at what cost? It's more expensive and take up more room in the bag.

Originally Posted by SlvrScoobie

I think with the 1.4X and the 300 F4 IS I think ill be pretty set. Now to convince the budget to work with me! (need lots before I get that!)

yeah, it's a 420 f/5.6 with IS that has awesome IQ... That's the route I took as it seemed like the least compromising option. The 100-400 was my other consideration, but I hate it's design and how big/cumbersome it is at 400...

of course if you get either of the f/2.8 tele primes then the 2.0 works flawlessly too, but yeah, budget what?