In a quest to lower temps of Z600's components, I installed Arctic RC heatsinks on the RAM. Despite removing memory fan and it's shroud, I am seeing average of 8 C lower temps on DIMM's and around 4-5 C lower temps on both CPU's. Not to mention that system is now generating less noise. Tipical HP engineering- you remove HP active cooling component that required godsknowhowmanyworkhours to be created and replace it with universal passive solution that is priced at 3,5€ a piece and you get better cooling for whole system. Yeah, that makes sense. 
To improve cooling a little bit further, I replaced stock slot covers with ventilated Fractal Design ones.
Z600 rework part 2: This time I am waging a battle against CPU temps. Although they dropped when I did my last modification, CPU temps were a little bit high for my taste. Cranking the fan speed was no go (80mm Delta fan on 5000+ RPM? No Thanks!) and there is zero alternative CPU coolers for this system. Second fan become my only option and fortunately heatsink frame was made out of stainless steel which gave me idea to use magnets to attach the fans. 
15 mins later I lowered my CPU temps by 10 C and as a bonus, RAM temps dropped by 3 C.