oil pan 4, sorry to bring this back up even though we couldn't convince you before, but all your beautiful (sounding, I can't see it) work makes me want to try again, since your project is just completely awesome.
So lowering volumetric efficiency...your static compression ratio is currently very high, and the heat ratio decreases at the higher temperatures that you probably have are mostly offsetting the efficiency increases due to compression ratio. For example, they say 17:1 is the limit for gasoline engines where you won't see any gain at all (full throttle, part throttle operation you can go higher since again, dynamic compression is lower). Sure gasoline is different from diesel but air behaves the same way when compressed in either
Anyway, the most efficient part of the engine cycle is going to be the piston's expansion stroke, and that is the only stroke where the engine is making any real power. If you have high volumetric efficiency, what happens is you're leaving more pressure behind at the end of the expansion stroke. Since you have a turbo, the turbo will pick up some of that energy, but as you mentioned it is very inefficient at doing so. However inefficient it is, it is the only way of recovering "mechanical energy" directly from the exhaust, and it does so almost "for free".
If you swap the cams so that you are now losing say 15% of your intake charge, your effective compression ratio drops to like 18:1, but you have the turbocharger increasing manifold pressure so the overall compression that the air sees is much much higher, probably beyond the point where the temperature and thus heat ratio have dropped significantly. Since the turbocharger is being fed less of an excess of gas, you'll have less backpressure, and the turbocharger will be working more off the otherwise wasted blowdown pressure.
Water injection cuts the temperature of the intake charge, but water has lower heat capacity to start with, and dilutes the oxygen concentration of the air so it isn't the best way to reduce temperature.