Let’s start from the very basic fact that comes from the ribbon microphone physics. The frequency response of the plain motor (without body and transformer) is only determined by its mechanical shape. The start of high frequency roll-off is determined by the distance from the front of a ribbon to its back around the frame. The smaller is this distance, the higher frequency the motor can reproduce. That’s why many motors have a sand clock shape with a thin waist.

From this point of view, the only advantage of a thin ribbon microphone foil would be the higher output signal level, which is reversely proportional to the mass of the ribbon. Say, if we use 1.2 micron foil instead of 2.5 micron in the same motor, the output level will increase twice, e.g. +6db. The downside will be the same amount of increase in output impedance of the ribbon, as the crossection of the foil is reduced twice. Increased output impedance will call for a transformer with higher inductance in primary winding to maintain the same bass performance. If the primary inductance is not enough, the combination of motor and transformer will act as high-pass filter, e.g. you will experience some bass roll-off. Sometimes it can be desirable though…

Now, if you have done some research on foil thickness topic, I bet you have found many forum posts and articles (sometimes with audio samples) sharing the opinion that motors re-ribboned with thinner foils tend to sound better. Do all these people experience the “placebo effect”, including yourself while listening to samples? Well, the answer is no. However, the secret is not in frequency response domain, but in a much more important spec – the transient response.