Is the stock ASUS P5K Deluxe cooling considered
trustworthy, or is one WELL advised to change it?

I remember hearing some comments by people about
things like NB / SB / MOS heatsinks with installations
that aren't actually even touching some of the chips
(e.g. the multiple FETs in a line), or with super-thick
gunky thermal tape so much that the overall cooling
is not so good, etc.

I also saw one article (which may have been biased)
that showed the back of the P5K PCB in the section near
the MOSFETs and CPU being VERY hot in the region of
100C under normal conditions.

I don't even know how well their heatpipes work in
practice whether the motherboard is vertical or horizontal
and whether there are mechanical/contact problems
with the NB/SB/FETS so much that it might not be doing
its job without reinstallation or replacement.

So I end up with some doubts about whether the
overall setup is "very pretty and theoretically impressive"
with the heatpipes, heatsinks, etc but maybe very
unreliable in the real world if the mechanical contact
and cooling is heat pipe action is not good.

Because of difficulty, risk, and not wanting to make things worse,
I would not want to tear off the existing heatsinks and
heat pipes if I can at all help doing it and they are
doing the job, but if they are actually blocking good
convection for the parts they supposedly cool I'd reinstall
them or replace them as needed/possible.
But probably wouldn't tear it ALL apart just for the heck
of it to replace their thermal tape with AS5 if there's no
well known major benefit to cooling of a large amount.

What can one do to cool the underside of the CPU area
of the PCB where there are bottom-mounted FETs and
the close confinement of the back side of the case
when you have no waterblock / heatsink designed for
that area, and your case hasn't got a back-side
fan opening? Any clever and cost effective ideas / products?