To get back on my point about idle vs load. Notebooks at idle should only be using what I call the passive thermal solution. Fans should not be running at idle.
So the heatsink through primarily conduction and convection and to a lesser extent radiation cool.
Add to that a heat pipes, pipe contains a relatively small quantity of a "working fluid" or coolant (such as water, ethanol or mercury) with the remainder of the pipe being filled with the vapor phase of the working fluid, all other gases being excluded. The advantage of heat pipes is their great efficiency in transferring heat.
Your removal of the back of notebook is all fine and maybe better at this point.
This is where you lose and mostly under load. Forced air cooling. If there is more air being forced into a system than being pumped out (due to an imbalance in the number of fans), this is referred to as a 'positive' airflow, as the pressure inside the unit is higher than outside. A balanced or neutral airflow is the most efficient, although a slightly positive airflow results in less dust build up.
You lose that when you open up the case. Also this final variable complements the effectiveness of the others.
Test under load.