So I've been using my 70-200MKII with my 2XIII and been getting good results, but now I'm thinking I want something a little more crisp and thinking about the Canon 100-400 or the Sigma 150-500. I can get them both for about the same price, but I want a little more reach and want to know if I used the 2X with the 400 to about 500-550 range will it be cleaner than the Sigma? I'm thinking the 400 since I can zoom back to a true 100mm as opposed to having a 140 when just using the lens unlike the 70-200 with the 2X. Plus if in a pinch I need a bit more reach I'll get that with the 400+2X. I know if I go to the full 800 I'll have some IQ issues, but how about the 500 range? Better or worse than the Sigma? Oh and I guess my numbers are off a little too since I have a crop body. Also, if I remember right the Sigma will not work with the 2XIII, right?