A lower-than-spec pressure cap is a band-aid for a fragile, leak-prone system. When a radiator builder says something like "Run a 7-pound cap", that translates into English as "I do crummy work; get your radiator somewhere else".
Lower pressure = more localized nucleate boiling at internal hot spots = poorer heat transfer out of the engine and into the coolant.
That is unless you are running waterless coolant, which has a very high boil point and so won't boil at hot spots even at atmospheric pressure.