I have the 100-400 L MKII and it is really lovely. I never owned the MK1 but I had used it a couple of times and although it is a good lens there was just something about it I didn't like and I really didn't like the push/pull zoom. I used to own a 70-200 f/2.8 L which was my favourite lens, it was just so sharp and fast but I knew I had to sell it in order to go full frame and that I'd need a 100-400 as it's replacement. I honestly thought that I'd miss the 70-200 and that I'd be disappointed in the 100-400 but I couldn't be more wrong. There is nothing I don't like about the MKII, I don't even miss the f/2.8.
I used to use a 2x TC with my 70-200 and whilst there was an image quality drop it really wasn't as noticeable as people say. Unfortunately a lot of people tend to repeat what they've read rather than what they've actually experienced themselves. If you peak at pixels then I could see a difference with my 2x TC. I once borrowed a 1.4TC and the difference was less but only slightly better and again only if you pixel peep.
I no longer own any TC so haven't tried it with my 100-400 but the MKIII TC's are even better than the MKII that I had. Will they be as good as a longer lens without a TC? No probably not but again, it's really only noticeable when you pixel peep.
I've not used the Sigma 150-600 but I did used to own the Sigma 50-500 and used the Sigma 150-500. I don't know how the 150-600 compares against them but the two lenses I used weren't great lenses. That much zoom required too much compromise on image quality, CA and focus speed. When they were in their sweet spot they were great but outside of that and they were noticeably poorer.
Don't get me wrong I know how frustrating it can be to look at your images on screen and think "it could be sharper" or "it could have more detail" etc but never forget that you are viewing your photos in a way that nobody else will.
Should you sell you MK1 and buy a MK2 or a 150-600? Yes. No. It's up to you. If you do buy a MK2 or a 150-600 will anyone else notice the difference? Probably not. Will you? Yeah. If you pixel peep.