The downstream force of the IP is an important factor for the performance of a 2nd stage.
In conjunction with other factors like the diameter of the orifice, the spring tension, the lever advantage, the seating force and the Diaphragm force it produces at a certain IP range and adjustment an optimal cracking and inhalation effort.
There is quite some difference the 2nds are reacting to a lower IP between an unbalanced and a balanced 2nds, but it is not that big as some might expect.
Usually translates in my tests a 3-6 psi IP drop into a 0,1inch/O2 increase in cracking effort in unbalanced 2nds and a 6-9psi drop into a 0,1inch/O2 increase in balanced 2nds when the 1
st stage is adjusted to an IP of 125 - 145psi (tank test pressure 3000 and 600psi).
So if the 1
st stage was originally adjusted to an IP of for example 140 and the 2nds adjusted according to that IP to 1inch/O2, and for whatever reason the IP drops to 110psi at full tank I would expect an increased cracking effort of about 0,5 1inch for unbalanced and an increase of approximately 0,3 0,5 inch/O2 for balanced 2nds.
That does not put in account the decreasing IP over the course of the dive.
So if the IP is wrong in this example I think one would feel the difference with an unbalanced 2
nd very clearly, but also with a balanced 2
nd.
It would be a different thing if the 1
st stage was adjusted on purpose to that low IP of 110psi and the 2nds were adjusted using that low IP.
I see also another problem in adjusting the IP quite lower than the specs of the manufacturers.
When breathing from a regulator the dynamic IP will decrease from the static IP. How much depends on the type of regulator. With an IP adjusted below the specs the dynamic IP will decrease more than normal.
This will increase the inhaling effort in the beginning of the inhaling process even more until the Venturi assist kicks in which will reduce, neutralize or override this increasing inhaling effort.
With a lower air flow (IP) I would expect a later kick in of the Venturi assist which could cause harder breathing .
Apart from that I think could a pretty low dynamic IP cause problems in an emergency if in greater depths for example a panicking diver has to be provided with quite more air than normal. This should cause a further dynamic IP drop.
Im not sure which low dynamic IP could be really a problem in general, but when a first stage in our flow tests shows a dynamic IP of less than 100psi, we automatically change the main spring.
That almost always solves the problem.
I think there are some good reasons to adjust the 1
st stages accordingly to their specs.