30MPG on the highway is quite good and will be difficult to improve by much, by adjusting/changing/removing parts.

The airbox definitely reduces intake noise. Its most obvious purpose is to mount the airfilter and ducting to the AMM so there are no leaks. When you compare the size of the filter and ducting on the 2.3L Volvo to other cars with much larger engines, the system is more than adequate, unless those other makers have seriously goofed. Unlikely.

The thermostatically heated intake is really only of benefit in cold weather to improve fuel vaporization before the engine is warmed up, which will yield a slight improvement in emissions and fuel economy during that period.

Some have suggested it speeds engine warmup, but that's hard to believe, given the heat capacity of air vs. that of 400lbs of iron, steel and aluminum, two gallons of coolant and a gallon of engine oil. Warmed air will have negligible effect on that mass, compared to the heat of combustion.
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Bob (son's 81-244GL B21F/M46, dtr's 83-244DL B23F/M46, my 94-944 B230FD and 89 745 (LT-1 V8); hobbycar 77 MGB, and a few old motorcycles)