Good question.
First of all I would think the ac impedance of the coil has to match the impedance of the detector's transmitter/receiver to get maximum power transfer to and from the coil. Enter FBS with 28 frequencies of bandwidth. Also the energy to and from the target is not necessarily a pure sine wave, in fact its likely pulse or other complex waveforms which can tend to complicate a maximum efficiency coil design.
Other factors which could affect coil efficiency are;
the core size and material,
number of windings,
winding DC impedance,
winding inductance,
capacitive component of winding/cable,
winding wire guage,
the coax cable shielding quality (copper or aluminum wrap shielding),
the coax cable impedance itself (at frequency bandwidth),
quality of coax connector (gold plate),
quality of electrical connection from cable to windings,
geometric shape of coil,
positioning of the D coils relative to one another,
and possible shielding internally in the coil are what i can think of off the top of my head.
My guess because of the complexity of energy to and from the coil and all the above factors, there could be a lot of imperical tweaking to get the maximum efficiency out of a coil. Probably one of the most important factors is the ability of the coil and cable to be immune or shield against environmental EMI radiation. The receive voltage from target eddy currents to the control box is tiny enough that any external EMI could raise the receive signal's noise floor resulting in a degradation of the signal/noise ratio.
Edit:
Wanted to add that the ctx 3030 has an edge over the eTRAC because of FBS-2 and its smart coil. The FBS-2 coil digitally processes those tiny target eddy currents right at the coil. That now higher voltage digital data which is much less affected by external EMI is sent to the control box.
In the case of the eTRAC and most other detectors, those tiny target analog signals are sent up the cable to be processed in the control box.
