More info:
Our coolant’s boiling temperature of 223° F (106° C) is when it is under 1 psi, which is why most cooling systems are designed to operate at 15 or 18 psi.
As the coolant heats up, it begins to expand and create additional pressure. By allowing that pressure to increase, the boiling point of the coolant is increased to around 257° F (125° C) at 15 psi. This gives us better performance from our cooling system as it can absorb additional heat from the engine without boiling.
https://www.cgj.com/2013/05/14/how-does-a-radiator-pressure-cap-work/
This article is suggesting using a pressure cap with a higher pressure rating to increase cooling system performance ... as long as you don't blow a hole in the cooling system etc ...
On an old car with a shabby cooling system using a higher pressure rad cap may not be a good idea ...