The boost gauge normally reads pressure within the plenum. Therefore if you are reading 12# of boost on the gauge and installed a second gauge at the turbo outlet, before the ic, you would probably read closer to 16# on a stock ic.
If you add a low restriction aftermarket IC, then you might read 13.5# at the turbo outlet with 12# in the plenum.
Now, because the turbo only had to make 13.5 at the ic inlet to get 12# in the plenum, the intake charge temperature would be somewhat cooler and you would make more power, potentially, even tho the plenum pressure was the same.
If you add a low restriction aftermarket IC, then you might read 13.5# at the turbo outlet with 12# in the plenum.
Now, because the turbo only had to make 13.5 at the ic inlet to get 12# in the plenum, the intake charge temperature would be somewhat cooler and you would make more power, potentially, even tho the plenum pressure was the same.