I was checking out a few days ago a Greddy radiator cap when I noticed that the piece is a 1.3 (127kpa), but my stock cap has a 137(kpa), my question is do I have the better cap or is the lower the kpa, the better, sorry for the ignorance it's just that i've never paid too much attention to how exactly the radiator caps function, thanks in advance guys!

 

higher the kpa the better.

Well kinda too high and it can cause problems.

But the fact that the stock cap is higher I would stick with the stock cap.

Even the "STI" radiator cap is rated at 1.3 bars.

 

Ideally, a replacement cap should match the pressure rating of the factory radiator cap. Maintaining a pressurized cooling system will raise the boiling point of the coolant as long as the system has no leaks. The cap will vent coolant to the overflow reservoir bottle when the system begins to exceed the rated pressure of the cap.

Raising the boiling point temperature of the coolant eliminates air bubbles in the cooling system. Once air pockets start to form, overheating is imminent because the cooling system will not be able to draw heat away from the engine where air has formed. If the hot spots form around critical areas (which they usually do) like the combustion chambers or near the exhaust ports in the cylinder heads, the heads can crack from overheating.

For street-driven cars, I wouldn't mess around with caps other than genuine Subaru parts. Going too high or too low with the pressure rating on the radiator caps may cause problems.