You can make a little more or less boost just depending on atmospheric conditions. The blower spins at a fixed rpm, unlike a turbo, so it has zero adjustability. It can only compress what it gets, and what it gets is more or less dense depending on temperature and humidity.
The catless x-pipe reduces the restriction through the motor. This is why it adds power. On a boosted motor, boost is a measure of the restriction through the motor. Take the cork out of the backside and boost drops on a supercharged engine. Again, boost is fixed by the blower belt drive and has zero adjustability, unlike a turbo that will simply spool more or less fast to make the boost set by the wastegate.
Now raise the boost to ESS' specifications and you should see more power. However, I would first want to know why my installation made less boost than boost on identical kits on supposedly identical cars. There could be belt slip or a boost leak. Sometimes it is just a bad boost gauge, so I would want to check that as well against another reference.