what "group incoming" pipe are you talking about?
changing boiler pressure (temp) is not a suitable way of changing your shot temperature.
The heat exchanger system in our coffee machines is already such an imprecise one due to it being a non-steady flow system most of the time, that adding a precise device such as a PID will do little to nothing for your coffees. All it will give you is deadband adjustment and a fancy looking digital display on the front of your machine, thats it.
I think youre trying to get more out of that machine than its fundamental design will allow you to do. Not to mention that all the brass components are so heavy that they take 20+mins to reach a steady state temp.
In my opinion, it wont work at all.