In my opinion they don't. If you only experience a used ZHP with worn down parts or put sport shocks in your ZHP, you wouldn't have the right feel for what they are. I tried some on my ZHP and when I refreshed the whole suspension it was perfect. So yes upgrades are in the eye of the beholder but you have to know what you are comparing with. If you have a worn out ZHP suspension and change it to a new one it WILL feel better of course and give that impression but the stock suspension was a work of art. Not just me, people that reviewed it raved about the balance between performance and comfort and it WAS specifically tuned for ZHP whereas Koni and Bilstein are tuning theirs more generically for sport and ZHP the same.
On the exhaust I owned that exhaust and the fine print said that there would be no power increase on ZHP. I just can't find the original documenation. Same thing with the E46 BMW Performance suspension it was also not considered an upgrade to the ZHP.