It's obviously not a simple subject.
Looking at this link, they explicitly state the box does not increase pressure.
There's a lot of processing goes on in a tuning box. Hence the 'high' cost.
There's a lot it can't do, as you say, turbo pressure, torque limiters, top speed limiter....
But, I can understand that intercepting the pressure signal can have an effect on the injection time, if the signal feedback is not [just] changed in amplitude, but time displaced. Yes, it's fooling the ecu.
However, if it were just effectively reducing the signal, would it cost £300+.
Details on how it works seem a bit vague but, lets see what else turns up.....