Spectral efficiency of with OFDM vs without OFDM?
The spectral efficiency of OFDM is strictly worse than that of a pulse-shaped QAM signal with the same rate.

OFDM requires a guard interval, on which no useful information is transmitted.

It is common to dedicate several subcarriers to pilot signals.

It is often not pulse-shaped (or, more accurately, pulse-shaped with a rectangular pulse).


Since each non-pilot subcarrier is essentially a narrowband QAM signal, you can see that its spectral efficiency cannot be better. However, the point of OFDM is to allow for low-complexity frequency-domain equalization, with spectral efficiency given a less important role.