We have had this subject come up before and I researched with the Aqua Hot technical folks and they said they found the lower pressure caps harder to find and went to a higher pressure cap for availability.
Keith, the reason your cap leaked did not have anything to do with pressure rating of the cap. It had to do with ageing of the rubber seal that no longer kept the fluid from leaking at the cap interface. The only reason to raise pressure in this boiler is to raise the boiling point and a 6 to 8 lb cap has more than high enough boiling point to be well beyond the high limit cut out of the safety Aquastat limit. The reason I bring this up because there are several weak points in the Aqua Hot designs that we have that tend to break and leak.
The cap itself, the hose that goes to the expansion tank (I went to silicone hose), and the steel to brass soldered interface that is between the riser fill pipe and the cap receiver. That interface is not sleeved and is prone to leak in time as thermal cycles stresses that joint.
Like the white plastic overflow tank on our Cat C12s, lower pressure and less stress makes this tank last longer, I run a lower pressure cap and my tank is original. Basically whether you use a 6 to 8lb cap or a 12lb cap, the boiler is fully wetted with fluid, the zones are free of air, and they flow properly. It is just at a lower pressure, the expanded fluid begins to flow to the overflow tank and back with heating and cooling...actually that is a good thing because it is less stress on the system.
Aqua Hot, in my discussion with engineering folks, their designs were actually going to a non pressurized system. Higher pressure equals higher boiling points, there is no other boiler operating parameter that is affected by the pressure rating of the cap. I have had cap leaks, hose leaks, the solder joint fail and have repaired them all and run the lower pressure cap. Aqua Hot used to ship the units with lower pressure caps. Hope this helps.
Later Ed