My S60 has a developing problem where considerable quantities of soot are emitted from the exhaust. Upon first startup, cold engine, the smoke is nearly entirely absent. When the engine reaches some particular temperature (normally at consistently the same point on my journey home/into work) the message area shows "Engine Service Required". Shortly after this point in my commute there's a long uphill, and if I take it in 3rd gear at around 2000 rpm there's so much soot flying out of the exhaust it causes the trailing car's automatic headlight lights to turn on (if it has the facility, of course).
I've also noticed that there's a periodic hesitation in the power delivery - again going up that hill in 3rd at about 2000 rpm, the power surges on and off slightly, every second or so. Generally the power delivery is poorer than it was, with the car sounding a little more "asthmatic/wheezy" than it used to
Fault codes read recently indicated fuel pressure issues, sometimes too low or high, and a code from the turbo control system (i think it was "6805 boost pressure control fault") but it doesn't cause limp mode (in its current manifestation)
A local garage said they didn't find any obvious intercooler or hose leaks but turbos aren't their speciality, and recommended another garage further away with more experience in forced induction
From the symptoms described, are there any causes more likely than others for the soot?