My wife and I just traded her 2010 335 xi in on a 2013 335xi with the M Sport package. It has the upgraded sound system which is Harmom-Kardon in the 3 Series and has 16 speakers like the B&O system. I have a 2011 535xi that came with the Hi-Fi Professional sound system.
After reading your post and while setting up the HK system for my wife's iPod music collection I tried my MP3 player with its music collection in her system. I noticed very little difference between the sound there and mine. As an audiophile (like you) I expected as much. With all the sound controls (treble, bass, equalizer) it is easy to tailor the sound just as you like it and, when alone, I love to crank it up. It is like travelling in a sound studio while riding on a cloud. It is the joy that the GT gives me the most.
In a perfect world, you would have a 2014 GT and a gently-used one side by side so you could compare the music in each sound system. While the HK system is not the B&O system, I would not expect there would be enough difference between it and mine to justify the cost differential between a 2014 GT and an older one with Hi-Fi Professional System.
And finally, I found this post on another BMW forum site that might be helpful:
I have the "HI-FI Pro" option in my 550 and in my soon-to-arrive M5. I think this is the best original factory sound I have had in a BMW.
A HI-FI magazine here in Sweden tested car HIFI a year ago, Audi A8 with B&O, BMW 7 with HIFI Pro and a MB S Class.The best sound was the B&O but
it cost 8 times more than the HIFI Pro system in the BMW, and some of the testers liked the sound in the BMW better than the expensive B&O. So best
sound for the money was the HIFI Pro in the BMW. The MB was not in the same class as the other.
I'm interested in home HIFI and have never heard a B&O that sounds better than other high-end products that always costs at least half the price of the B&O..