Yes great news! Glad you got it sorted. Concur that it is a mystery how it could have been out of adjustment, perhaps for a long time, but it makes sense based on the fact that it was down on power yet able to make full boost, plus the total absence of smoke under load.
Could be it was a rebuilt pump at some point that was calibrated to the wrong specs -- for example with max fueling set to non-turbo values?? If the anti-tamper paint had not been broken then hard to guess any other explanation.
Based on your mention of a crimped-on collar, you are referring to what is most commonly (on this continent at least
) nicknamed the "smoke screw". I think I referred to it as the fuel quantity adjustment earlier. It sets the level of the entire fuel delivery curve for the engine, and then the LDA and the governor (the latter being manipulated by your right foot) make adjustments from there. Thus, yes, adjusting this screw will cause the low idle speed to change dramatically as you mentioned in a different thread. To compensate for that, once you have the smoke screw set to where you want it, you will need to make two other adjustments: one to the position of the low idle throttle lever stop screw (see photo and info ngoma posted in other thread), and additionally you will need to adjust the position of the ball stud on the cold start advance lever that kicks the throttle lever up to a fast idle position when the cold start advance system is in "cold" position. It will need to be loosened and adjusted towards the rear of the car. This will probably take a few days of cold starts to get where you want it, aiming for about 1000 rpm idle IIRC when cold and then 750-850 when warm.
Further -- and this part is specific ONLY to Volvo cars, unlike the steps above that apply to all engines using the Bosch VE IP -- you *may* also need to adjust the length of the turnbuckle style linkage rod that runs from the bottom of the throttle cable bell crank spool back to the actual throttle lever (and/or, adjust the position of the ball stud on the throttle lever that that rod attaches to). This is because, as you turn the low idle adjustment screw outward to reduce low idle speed setting, eventually you may reach a point where the throttle mechanism is no longer resting against that screw and instead being stopped by the throttle bell crank rotating to its minimum stop position. This is not desirable - you want the low idle screw, not the bell crank, to set low idle speed and you want an air gap clearance at warm idle between the bell crank and its stop. The factory greenbook gives a clearance spec for all this. It's due to Volvo's funky two-part progressive throttle control mechanism that they adapted to VW's engine, resulting in the additional fiddly adjustments, but it all works very nicely provided you get it dialed in.
May want to finish final adjustments to the smoke screw before undertaking all this. (aiming for just at the point of a slight puff of smoke as you push pedal to full load, and then it should clear up entirely once under steady full load, don't want any continuous black smoke) Since any further tinkering with that smoke screw will cause you to have to redo your adjustments to idle speed and related parts again.
Welcome to the board! So now on to the cambelt change?
Unless you are already familiar, may want to read up a little on this forum and/or in Volvo service literature regarding the particular tools and techniques needed.