I recall another debate on this recently (flyer?).
The Rotax wins a bit on efficiency by using a higher comp ratio, but loses that by having smaller pistons bobbing up and down more often. The SFC of the two engines ends up within a few % and probably closer.
That's assuming the Lyco/Conti engine is leaned correctly i.e. peak EGT or LOP. Historically, the PPL training machine has not been teaching engine management so some 20-30% of the fuel is wasted.
Years ago I saw some analysis on engine efficiency and the old engines beat every modern car petrol engine on SFC.
There is also no doubt the old engines are more reliable than the Rotax. They have fewer bits and no gearbox. A geared engine running at a higher RPM will only ever be able to approach the MTBF of a simpler engine running at low revs.
The other reason (other than mixture mismanagement endemic with Lycos/Contis by "traditional" pilots, as a result of poor training and poor engine instrumentation) why Rotax-powered planes use less fuel is because they are smaller! The fuel flow for a given IAS is mostly a function of the cockpit dimensions. The cross section is the biggest thing. A tandem 2-seater will burn probably about 1/3 less fuel than a side by side 2 seater. Most of the Rotax powered planes have really "intimate" cockpits compared to say a TB20 so no wonder they burn half the juice.
The RPM v. torque argument is spurious. The prop has to absorb the engine power and the thrust comes only from the power.