I have some 320's that sound better than FLAC's and it's pop music. I would just add one thing; it is the quality of the mastering, for sure, but I suspect it is the ripping process as well.
editinig just for the sake of the conclusion; I think 320 is 'big enough' if you do everything properly.
I have only dipped my toe in to the lossless coding world. I have a couple of purchases from HD tracks of CD's I own and digital rips from those CD's as well as purchased copies of digital files on those same tracks. That's a long winded way to say I have the same music in everything from an iTunes file to an HD Tracks flac file that costs 10 times as much.
While that doesn't make me any sort of expert, it means that I can tell you what my own personal experience has been comparing the formats using the same songs. First off, I believe there's a difference between the FLAC file and the old and fairly low rate iTunes file. Quantifying that difference is hard because if I wasn't sitting in my listening chair in a perfectly quiet environment and straining at gnats, I might not be able to say much.
I also have ripped a lot of Blu-ray music and CD based music. I have converted all my physical media to digital files. That process took me almost a year. I experimented a lot with bit rates to see if I could hear the difference between different choices. I'd like to be able to report that the higher bit rate choices always made a difference. Sadly, most of the time, I couldn't tell much of any difference.
What I ended up doing was ripping at the highest bit rate/sampling rate for everything because even if I can't hear the difference right now with the equipment I own, I am future proofing my library against the day when newer DACs or equipment choices may make a difference.
IMHO the advice that 320 is enough, is pretty solid. The biggest difference that you can hear has also been noted: the source material itself.
The source material itself and its original recording and preservation makes more difference to how a song will sound than anything other aspect of the song. No amount of digital oversampling will save a song that was poorly recorded to begin with. And there are 1,000's of examples of poorly recorded music that is ending up on "remastered" or HD media for sale. Read some of the critiques of albums that cost $40 bucks on HD websites that have exactly the same content as a CD.
Caveat Emptor.