180 or 190 thermostat?

The point that I keep trying to make is that the pressure is NOT constant thru out the whole system. Put your thumb mostly over the end of a garden hose with the valve wide open and tell me that the hose didn't get slightly larger under your hand. An increase in local pressure due to the restriction of your thumb did that. Same thing happens between the pump and the t-stat. The t-stat is effectively your thumb partly blocking the exit and raising the pressure of the coolant behind it.

The lowest pressure in the system is between the t-stat and the pump inlet, and that is the cap's pressure. Between the pump's discharge and the t-stat it is at a higher pressure due to the restriction intentionally provided by the t-stat. The goal is to increase the pressure in the block so that cavitation and localized boiling are less likely to happen. Even if it is only a couple psi above that of the cap it has bought the system a 6°F higher boiling point. Look at what it does for you if it is 10 psi above the cap.

This all about heat transfer (rejection) rate. Slow the coolant down and you will reduce the transfer rate. The rate of heat transfer is directly proportional to mass flow rate. <- I stole that from here: http://www.overclockers.com/water-cooling-flow-rate-and-heat-transfer/ Found that looking for some simple math to prove it. That formula is:
Q = M x C x Delta T
Where:
Q = heat transfer rate
M = mass flow rate
C = Constant, specific heat of the coolant
Delta T = the change in temperature of the coolantUgh, Now that I see it, I recall that formula from Thermodynamics.
Notice that none of the factors are fractions or divisors. That means that any change in them has a direct rather than inverse effect on the thermal transfer rate. Increase any of them and the heat transfer rate goes up. Decrease any of them and the heat transfer rate goes down.

The factory knew about how much flow they needed to have to keep the engine cool and designed the pump and the drive ratio to work together to provide enough flow. Why do those who go to underdrive pulleys sometimes have over-heating troubles when going slow? They don't have enough coolant flow. That is the only thing that they changed.