Good afternoon Harley
I have both an older K3 (serial number 670) and a new K3S so I've had the chance to compare both in an Urban environment and a Countryside one. The only mods I've done on the K3 is upgrade of the main DSP board.
In my Urban environment, I honestly did not notice any difference between the RX performance of either radio. I have high electrical noise levels and on 80M that runs at about S8.
In my Countryside environment then I did notice a significant difference.
The 80M noise level is S2 (as a comparison) and the K3S was significantly better at resolving weak signals (I'm a QRP operator so this is important to me) The K3S also sounded 'cleaner' when listening on headphones.
In both cases I operated CW, SSB and DATA contests.
I know that's a bit of a rough and ready comparison, but my one line summary would be:
73 Bruce G4ABX