[morzh]

IF was chosen low for a reason. It has to do with selectivity and not really the efficiency. The idea is, the selectivity is way better at lower frequency, as the distance between station (10kHz) is 10% at 100kHz, or 2% at 450kHz, but 0.2% at 4.5MHz. THis is why the TRF radios did not really go into the short wave bands.

Efficiency in RF transformer is not that important as not much power is transferred: it is pretty much the voltage conversion, and is chosen out of consideration for the type of tubes or detectors etc used.
Efficiency is important where the power is transferred. NOt only voltage or only current.

And in IF transformer the most important part is the resonant one. But the frequency choice was the compromise between selectivity and quality (too low a frequency will not give good sound quality, itself eventually venturing in the sound frequency band). Plus other considerations. Like triodes could not amplify high frequencies well, but being able to oscillate at them. And then to amplify you need to lower the frequency.