I think Haystack pretty well nailed it. Just to try to explain it in a different way, you have several factors going on in a cooling system. When you refer to "heat transfer efficiency" I see that as being a combination of several things. The inherent design of the engine block and components and radiator and how well it disperses heat. The coolant itself and whether it is water or glycol changes the BTU/pound of heat it can carry(glycol carrying something like 90% of pure water). How efficient the rad is in terms of overall airflow which gets into fan capacity and what the heat transfer of the cooling fins is per sq/ft or whatever. So all other components of the system being equal and back to "heat transfer efficiency", the increased pressure in the system increases the boiling point which reduces problem areas inside the block which would start to boil at a lower pressure. The increased temp of the coolant carries more BTU. That is what cooling is all about is removing the BTU. One pound of water raised one degree F is one BTU. So if the water temp is higher it is carrying more BTU so "heat transfer efficiency" will be greater. The higher the coolant temp in the rad the greater heat transfer to the air on a hot day. This is known as Delta T in the formulas dealing with heat transfer. For example in determining how many gallons per minute you need to cool your engine the formula is quite simple - gpm=total btu of water jacket heat rejection divided by 450 (factor for gallons per minute to pounds per hour x 90% for glycol) x Delta T. So if your coolant temp leaving the engine is higher, you get more heat transfer to the air(Delta T) . So for example if Delta T changes from 10 degrees F to 20 degrees F, not that hard to do, your gpm required is half and/or and ability to cool the engine doubles. So to reflect on your question, increased system pressure results in possible higher temperature without boiling, which can significantly increase the Delta T, so I would say that qualifies as increased "heat transfer efficiency".But...
Does the increased internal system pressure have any effect on heat transference efficiency?
I am speaking of this cooling system knowledge coming from a heavy equipment and farm background and more recently with an old boat with heating problems and more time on my hands. The engine is a 6D14 Mitsubishi NA in line 6 with 6.55 liters or 400 cid and 120 hp. It is keel cooled which means instead of a rad it has about 40 feet of 1 1/4" K grade rigid copper pipe in the sea water under the hull. I asked hundreds of people about where to start on this problem, including engineers at some of the heat transfer specialty businesses in Edmonton that build power generating and pumping skids for the energy industry. Without all the technical specs they would get from the design engineers for engines and cooling components, they just glaze over and do not have all the numbers to plug into the formulas and it ends there. So I started reading and compiling some of the key data factors in every cooling system. Start with the water jacket heat rejection number from the manufacturer. Which for my 6D14 is 138,000 BTU. But does that include the heat load from the water cooled exhaust manifold(required to lower engine room temps and therefore engine air intake temp etc etc.) and the transmission, and the hydraulic system cooler? I have learned that NO it does not. So I approach total BTU generated from how much diesel is burned per hour. So for my engine 120 HP, generating 16 hp hrs/ US gallon of diesel?? (all engines are in a range of about 15 to 18, newer higher tech being closer to 18) I come up with 7.5 us gal/hr. at EPL. Diesel has about 138,000 btu/gal so X 7.5 = about 1,000,000 BTU per hour. Average old school engines like mine disperse heat about 1/3 out the cooling system, 1/3 out the exhaust, and about 1/3 gets turned into rotary energy and friction. So if I have to get rid of 330,000 btu from the rad, plus say half of the btu from the water cooled exhaust manifold, plus 5-7% of the rotary energy in the transmission, and some % of hydraulic system heat generated, I am well over 500,000 btu total heat to be removed by the keel cooling loop. Vastly more than the water jacket heat rejection figure of 138,000 btu published by Mitsubishi and what I suspect my keel cooler was designed for. I got into several heated debates with keel cooler manufacturers and designers that only looked at the 138,000 figure. One guy told me "I have been doing this for 30 years, are you questioning me"? Well yes I am, and that is why my engine is overheating and always has. So I proceeded to build a test bench to measure the gpm flow of my water pump which is the other huge factor. After rebuilding and machining the impeller clearance to .020" it was really close to the manufacturers spec of 58 gal/min at 8 psi. So now I can do some flow calculations through my 1 1/4" cooler pipe with a dozen 90s and Ts and fittings and restrictions. Not even close!! It would take 40-50 psi to push 58 gpm through that maze of cooling pipe. The engine spec is 15 psi max internal engine pressure. That rules out a 9200 Hypro sprayer pump that could push 58 gpm but at 40-50 psi. And at those higher flow rates the maximum feet per minute flow rate in the pipe of 2-8 feet per minute is exceeded. So right there I know the existing cooling system is way undersized for handling 500,000 btu with 58 gpm and with total restriction on the system of less than 8 psi. So the only solution is increase the size of the plumbing to all 2" and shorten the length and restriction of the system. I have to install my new Fernstrum 2" diameter pipe keel cooler this spring so have yet to see if it will all work. Also I have determined that the factory thermostat and housing is part of the restriction problem so am building my own temperature control manifold patterned on an Amot temp regulator used in industry for this exact purpose - better control and full flow in bybass mode. I am using a 3406B 190 F thermostat for ample flow.
Cooling system problems used to contribute to around 40% of engine failure problems. It all seems like a pretty big mystery at first glance, but when you break it all down to what one BTU is, it all starts to make more sense. And if the design engineers would allow a little more wiggle room in the capacity of cooling systems, we all would have a lot less expensive problems.