In thermodynamics there is no contact pressure. You are either in contact or not. When you have two material in contact, you can transfer heat by conduction. If you apply enough force you can squeze a foreign material between two others. SO what you are doing with contact pressure is squezing air or thermal paste of the bond between the die and the heatsink.
It depends on your mounting technique. For most HSF yes it would *if* you lap a large amount off. Higher pressure = better contact = better transfer. If your HSF surface is bad it would probably be worth a little pressure loss however
If anyone can lap off more than 1mm, I'd say that's WAAYYY excessive. Anyhow, contact pressure is an important factor in heatsink mounting but nowadays I feel it's a tad too much compared to old pentium HSFs. 12 to 24 pound-force (try holding a 5 to 10kg mass in your hand) says AMD.
that's what you really want to avoid. if your hs is floating on ASII instead of just filling in the cracks then youve decreased the thermal transfer rate.
i'm not a believer in lapping. only time i would lap is if the hs was dented or bent in such a way that good contact between hs and core was impossible.
i don't believe in polishing, which is what most ppl that lap seem to do. polishing is bad.
you kinda hit it on the head when you said it lowers pressure, think about it, when you polish youve significantly increased the area that is trying to make contact w/ the core, like flats on a racing car. this would ideally be a good thing, but then you got that dab of ASII in between, asII being the race car flats equivalent of water. what happens to that race care w/ it's flats when water is thrown in the equation, because of the absolute surface area of the flats as opposed to treaded tires, the tire floats on the water.
same w/ hs's and cores. you can definitely polish so that your hs would float,
the purpose of asII is so that you won't have to lap.
if you are going to polish your hs, i strongly recommend using almost no asII.
that's why you polished it right, to improve contact between hs and core?
Well theres no use saying lapping in not good or benificial I would say that 90% of all overclockers have had great succes lapping, if you polished to a mirror shine it is still better then the original finish there was a post a while back and this person polished first and had better results then lapped to 600grit and then to 1500. both 600 and 1500 gave simular results but all better then the original finish. I think most people now are lapping to 600 as this makes the surface flat and leaves enough hills and valleys so that the thermal compound will have full effect. Note 3\4 of lapping is making the surface flat and this is very important. If you have ever had a HS flycut at a machine shop you would see just how out of wack they can be, i just got a Gladiator\Goldengate and the finish on mine was horrible without lapping i'm sure it would be in the range of 3 to 5 C hotter. another note evryone will get different results lapping but 95% will get better results if done properly. To this day ive never heard of someone saying they had worse results after lapping. And as far as Pressure goes the amount that you remove lapping will have such a small effect on the amount of pressure being applied thet you would never know that it was not original equipment. Also lapping has no ill effect if your using springs to hold down your HSF like Hoots article on the front page.