Also I think one of the main points here is that even though the iPhone shoots 4k, teh Alexa's 1080p still outperforms. This is again, something some of us may know, but with the race for everything to be 4k it's important to have some solid side by side to show that it's not always better. And I'm talking about even real 4k cameras that are for production. Sometimes 4k just doesn't solve things and as someone else pointed out in the comments here, it's good to have that to take to some clients.
I use that lens (as well as the sigma 50-100) on my c100 mk1, and I haven't really experienced this issue. It's possible you just have a bad lens (or, some of these can be recalibrated using software). I never personally trust focus peaking (it has ruined shots for me in the past many times). In general though the Sigma Art series is one of the best lenses for sharpness and built/price I've found, even fairly wide open. So in general, I mean, I would say it's NOT the lens.
If you're shooting at F10 or so, you should be able, for the most part, focus by sight using the viewfinder or LCD and just try that. Also, haze can affect the shots as well. Have you experienced this issue in other locations?
Hmm... Well, depending on the footage you should almost always apply the LUT first. But that also depends on the LUT. For instance, if you shot in RED RAW, first step would be to de-LOG it via a Red Raw to Rec 709/etc LUT. If you shot in Canon Log, do the same thing. Do that to all the clips first. Then apply your color grade/curves/corrections.
Then at the end if you want you can add a finishing lut, which is more of a look. This would go in the adjustment layer. Or I suppose you could do one adjustment layer that's just the start LUT, then do all the normal fixes on the individual clips, and then add a second adjustment layer that's the finish lut (faded film look, whatever).
Also remember that LUTs don't always need to be used. Depending on the footage you might end up crushing all the blacks, or blowing out the highlights, like you mentioned.
My guess is your issue above is being caused BY the LUT.
In general most software runs pretty good on both platforms.
Like Logan said, use whatever you're comfortable with. However, Adobe CC is basically built to do editing, color correction, audio, and SFX work, all in one suite, and with dynamic linking.
In my experience though, Adobe software runs worse on Macs. I think it has something to do with the various permissions etc that exist within the Mac OS and how it handles cached Adobe files and such. Where I work we have a number of issues with Adobe CC on Mac that I don't have on my custom built PC at home with the same software.
PCs are going to be a lot cheaper in general, also. And you wouldn't have to buy an external GPU, etc.
Also, I would like to add that based on my experience with Avid at my current job (we use Macs), it is by no means a no problems ever piece of software. We've had many crashes and funny error messages. I'm not saying it's better or worse than anything else, it's just I would never describe Avid as "no problem". Nothing is. Backup and duplicate. :D
For what it's worth I have edited a feature film on Premiere. It's entirely possible and anyone that says it's not doesn't really understand or like premiere because they're probably used to something else.
I work at a place that's 100% Premiere, which is great for me because that's what I use for my own projects as well. I know it and I like how much you can throw at it (codecs, effects, etc) and it just works.
Avid is great if you really know how to use Avid. And hollywood is changing and there are many places using Premiere. My advice? Learn both.