I did a significant overhaul of the stereo as well, but kept the stock HU.
I can only compare it to the stock setup which was awful, not an aftermarket HU, but I'm very happy with how it sounds even with the stock HU. An aftermarket HU would almost certainly sound better but I'm unsure by how much and I prefer a stock look on the dash. I've since looked at upgrading to the Kenwood HU's that don't require a bezel, but thats another £1400 installed (with DSP re-tune) so don't think I'll bother. I'm an apple user and the fact that carplay only works with Apple Maps (not google maps) was also a turn off. If I could rewind, the main change I'd make is adding more sound proofing to the back of the door cards as I do now have some vibrations at some frequencies and high volumes (they weren't there when I finished the build). Maybe a summer job this year.