Also, check the Audio threads on the forum to get some more insight.
So far I've disconnected the center speaker on the HK (improve stereo imaging), replaced the rear speakers with JL coaxials for a smoother top end. Next up will be replacing the front speakers with something that has more substance in the mid bass. Personally there's tons of low end from the subs but feels like the 300-600hz range is a little muted so hoping to recover some of that from the upgrade.
Granted, these are opinions of myself and others. I didn't end up paying the additional $1675 for the upgrade myself -- the car was already optioned with it when i bought it second hand so i didn't feel weird swapping out a 'premium' system. If you end up specifically ordering it, and still swapping it, then it feels like a bad investment.
If you don't ever plan on going down the custom route, FOR SURE get the HK because the regular Hi-Fi stereo is hot trash.
Take a listen for yourself and see which camp you align more with.
Some bathroom reading:
Dr. Strangelove, or How I Learned to Stop Worrying & Live w the Harmon Kardon System