Fuel injection and other "engine management" related software ... Volvo updates the software on the cars all the time, when was the last time any of the cars had been dealer serviced (FOC software is updated under the "service promise" every time the car is dealer serviced,) the cars could be running different software levels.
Plus
CAT, MAF, valve clearances .... most of the stuff you would check at a service fuel filters, air filters etc
Where to start.
not exhaustive and maybe not relevent to the V8's but something to think about.
Edit, also have you had the codes read using VIDA, there may be a pointer there.