When Bill recently did the timing belt on our engine, he used a good belt tension gauge, and a dial indicator to re-time the injection pump.  He also replaced the tension and idler pulleys.  They weren't expensive, and are critical bearings that are really easy to change while you have the belt off anyway.

If you look at the shop manual, one of the last steps is to reset the timing of the injection pump.  That is an important step that people seem to frequently skip. The camshaft and the crankshaft are locked in place (6mm bolts work as well as the $pecial pin$ from Volvo!) and as you tension the belt, the shaft of the injection pump will rotate--at least a little bit.  If you don't reset it, fuel injection timing will be off, and that is a bad thing.  Late injection will result in unburned fuel and reduced power, early injection can seriously damage the engine if the fuel ignites significantly before TDC. The adjustment is a little fiddly, but not hard.

It might be our imagination, but it sure looks like we accumulate a lot less soot on the hull after this job. Could fine tuning  the pump timing have helped with this chronic problem?

This is a very important project on these engines.  If the belt breaks or gets loose enough to jump a couple cogs the valves and pistons will hit each other and in seconds your engine turns into a very expensive mooring anchor. It's worth doing better than just eyeballing the belt tension, which is really high and quite hard to do by feel.