I recon i removed perhaps 40kgs from mine without touching anything thats actually visible, nor altering the comfort at all.
AC was broken when i baught the car so i removed the compressor, its bracket, condenser and pipework, as well as all the engine bay wiring from it, ditched the viscous fan and its bearing and removed the foglights as one was corroded thru and the other had all its mounting lugs broken off. I also binned all the random plastic covers in and around the engine bay, and removed some extraneous pipework for the evap system.
I then binned the CD Changer and its huge metal support bracket (this weighed a stupid amount on its own!), the OEM rear speakers and the hopeless OEM amplifier.
A portion of that will go back in when i finally sort out a subwoofer though.