Distortion.
The FET in the electrets capsule has 10dB negative feedback due to the insertion of a source resistor. This lowers the distortion in the FET at high sound levels. 

The unclipped maximal output of the whole electronical circuit is 5Vrms or +13 dBv into 2 x 2,8 kOhms mixer input impedance.

Most mixers can handle up to 1,5Vrms when its gain is set at minimum.
The mixer input saturates therefore BEFORE this driver circuit does.
The total distortion of the electronics remain very low until mixer saturation.

C5 and C6 cannot cause distortion at high audio frequencies in T1 and T2 , as R12 and R13 then limit the output currents in T1,2.
At 20kHz the maximal unclipped output  is still 2,5Vrms (+7dBv).
The small-signal audio bandwidth  (measured at 10mVrms) is <10Hz => 150kHz @ -3dB. 

        The electrets capsule max. output.
In an electret microphone capsule like this one, the current drawn by the internal FET typically is fixed at only 0,35 mA. It cannot be changed. Therefore the capsule cannot deliver more than 0,7mApp x 2k2 = 1,54Vpp = 0,55Vrms = -6dBv into one 2k2 load. This modified electrets circuit however has 2 balanced outputs and can drive the mixer input nearly fully (2x0,55Vrms = 1,1Vrms). 

        HF immunity.
Strong HF transmitter signals picked up by the microphone cable cannot cause distortion in T1 and T2, as C6 and C5 short them to ground and R12 and R13 weaken the remaining signals further.