Mga Bro.
In my opinion about this radiator cap sizes/configurations and pressure rating. Share ko lang my knowledge becoz im into 15 years in the manufacture/design of radiators, sa RADPHIL, SALAMA Rad in KSA and now in SERCK-UAE, makikita nio sa market ibang size ng cap say:
A) Nippon Denso Rad Cap = 1.1 kg/cm2 (ginagamit ito sa mga Plastic Tank Radiators, OEM)
B) Common Rad Cap = 1.1 kg/cm2 (gamit ng mga Brass Tank Radiators)
**A and B will have same performance. You may ask why ND designed new cap, its only a market strategy that they will only have this kind of cap.

Going to cap rating pressure 1.1, 0.9 and 0.5 kg/cm2, i can share some thermodynamics properties.
1.) 1.1 = boiling point is 121-123 C
2.) 0.9 = boiling point is 116-118 C
3.) 0.5 = boiling point is 110-112 C
Note:
In making a design for radiator we usually assumed in atmospheric condition, so 100 C as max. temp. of the cooling water, then droping it by 5 to 7 C. Sa Pinas we set the ambient air *35C pero dito sa Middle East we set our design Air Temp. at 50C. This will equate to more heat transfer surface area.
Causes ng Overheating Radiator side :
1. Cooling Tubes are plugged with dirts.
2. Cooling Fins are plugged with airborne dirts and damaged fins.
3. Common case, we always look for increasing the number of rows for better cooling, but the electric cooling fan will suffer due to increase in resistance, yung volume of air will not be sufficient. Note on air flow direction: from Front Grille- condenser and then the radiator - Cooling Fan. Upgrading your Rad from 1R to 2R you should consider upgrading your cooling fan (Condenser Side).