[v8volvo]

Seems fairly unlikely it would make a major difference. Maybe a little bit of marginal improvement at the very top of the RPM range when exhaust flow is greatest, but most of us don't spend a whole lot of time running our diesels up at those RPMs.

As noted elsewhere, although there may be a few marginal gains that could be achieved by fully optimizing the NA D24 engine, far and away the greatest performance and economy boost would come from converting to a D24T. If your engine is already out of the car it may be a golden opportunity! Although a good running fresh and healthy D24 can be a pleasing engine to operate as well. But any gain from things like bigger exhaust, overbore, cylinder head work, etc, is going to be at most a percent or two improvement. Probably not enough to even really feel. By contrast, changing to a stock D24T engine in a 240 is a difference you would feel in a huge way.

Forced induction through turbocharging seems to be the unquestioned path to improved engine efficiency and power for diesels..... hence the almost total absence of any non-turbocharged diesels produced by OEMs in modern market, i.e. for the last 30 or so years, outside of agricultural/industrial applications (and even there they are rare). I personally cannot speak to the theory of why diesels benefit so hugely from forced induction (like gas engines also do) yet have relatively little potential for power/efficiency gains through other traditional means that usually have great success in adding power without forced induction on gas engines. By those I mean things aimed at volumetric efficiency, like bigger exhaust, intake, camshafts, cylinder head port/polish, bigger valves, etc. But I would speculate part of it may be that the limited diesel engine speed range, usually topping out at 5k RPM or less, leaves minimal opportunity for taking advantage of upgrades like those mentioned above. A gas engine can spin faster and make use of changes that let it move air in (and exhaust out) more efficiently. And its rev range can be increased almost without limit due to the homogenous charge (air and fuel mixed before entering the cylinder) -- the only limit to engine speed is imposed by what the rotating assembly and valvetrain can withstand, and those too can be modified as budget allows to even let a big-block V8 rev to 8000 rpm or more by use of exotic light/strong metals like titanium..... Meanwhile, a diesel engine by contrast is speed-limited not by inlet/outlet gas flow, but rather as I have always understood it, by the diesel combustion process itself -- how quickly the compression ignition cycle can burn the fuel. And perhaps to some limited degree also by the necessarily heavier mass of diesel pistons/rods/crankshaft that cannot really be lightened much due to the diesel compression/combustion forces they are subject to. So even if you could bolt on a massive exhaust, fancy high-flow intake/exhaust manifolds, and an artfully ported and polished cylinder head with high lift camshaft, you'd never get the engine turning fast enough to create the kind of intake/exhaust airflow rates where those things could really make their difference. At least that's the version of it that makes sense to me.