Trouble with doing the chipping is that you can't use the power for extra speed unless you then increase your prop size (probably more pitch). That engine already turns at 3500rpm to make it's power so I don't think spinning it faster would be a great idea, so more power means prop changes to use it.
Simple thinking suggests that chipping a marine diesel without adding more boost will just add hp at the expense of increased fuel burn. It could be however that the turbo might be able to provide the extra boost but how is that managed when you 'chip' the engine? You then need to know whether the cooling system will cope with the extra hp production, especially if it is just adding more fuel without increasing the boost/air supply. You could check the cooling system parts compatability between your D6 350 and the higher hp versions to see if maybe the cooling system has the extra capacity already. Bottom line is that you might be better off asking Volvo if your engine can be reprogrammed for the higher hp (370hp I guess?). I would be pretty confident that you won't get to the 435hp of the upper limit D6 with just a chip change. So the chip might be an OK idea but time to do some homework on what it actually does and whether the engine can handle it in tems of cooling. I would guess that the 350 and 370 internals are probably the same so wouldn't be too worried about that aspect unless the chip is claiming a lot more hp.
Final comment is that a 5.5l engine making 350hp is a pretty solid 64hp per liter of displacement which is a decent recreational rating and even OK for light commercial use. Take it to the 435hp rating however and then you are nearly 80hp/liter which is not really a recipe for a long lasting engine. Modern injection systems certainly help but these engines aren't built super heavy like older diesels and there is only so much hp they can produce before needing rebuilding. Used to be that you counted a marine diesels life expectancy in gallons of fuel it can consume between rebuilds and I don't think that the picture has changed that much. Make more hp out of the same engine and then you get to rebuild or replace it that much earlier, plus you need to service everything that much more often.
