OP - for future reference, the FICM's will not do well w/ bad batteries and/or alternator, so make sure your electrical system is healthy! Even the new FICM you had installed (actually it was a reman because they aren't making new ones anymore) is susceptible to low voltage. Better than installing a new one, several shops will reflow solder and upgrade a few "under-designed" parts. I like FICMrepair.com as a very good combination of quality work with reasonable price. It is half the price of a remanned FICM and will hold up better. SWAMPS is good, but a little more costly.
Lots of us that have quite a few years of 6.0L experience can say for certain that the coolant filter has helped. On another forum I have pics of sand that I pulled out of my system. If you go to the Dieselsite web site, another forum person has pics of his filter with what he pulled out of the system. The filter is a good idea, but it certainly is a bypass design and is not a "cure-all".
As an FYI on coolant pressure. I have been monitoring my degas bottle pressure for a year (electronic sending unnit to a cab gauge). It runs 4-7 psig, depending on the coolant temperature and the liquid expansion when it heats up. Typical pressure is 4-5 psig at 190*F. One variable is how full is the degas bottle. If it is overfilled, the pressure will increase above the values I gave because there is a small vapor space to compress when the liquid expands (liquid is incompressible and can expand a fair amount when it heats up by 100 degrees). The most I have seen is around 10 psig under hard accelerations, and that is just a transient pressure.