Well, I didn't hear what I expected.
Right now in 2 identical systems with boxed E6550 Cpus - one with 3rd party unknown grease is running at 850 RPM and the on with original thermal pads is running at 1000 RPM. The single variable seems to be thermal paste.
Systems are running with same BIOS version and settings with changing the same hdd. IMO amazingly big difference! I can't believe it myself too but to do more precise tests isn't easy.