When it comes to Polyester and Polypropylène in the high frequencies we are talking about, the loss factor is really different on the datasheets I read.
Polypro is pretty flat impedance in the ear frequency range. Not at all for a polyester, which may explain for some a difference at listening according a specific tweeter, perhaps I'm wrong, let say the difference is subtle but big enough to notice it.
But again while some non polar lytic can be worse than a MKT, they are at the same time nearer from each other vs a polypro.
BUT Smooth foil non polar lytics which have a much flater impedance in the audio range than MKT, and also lower at treble frequencies...
This should partialy explain the difference one can hear differences, as also the construction and cap technic assembly according some litteratures.
You can hear sometimes as said above the difference between two speaker cables which have a huge capacitive, resistive, difference. There is also the amp. So I will trust myself knowledgeable people as AllenB and very experienced ones when they say price of the cap is not directly coralate to an upgrade. One has to know the effect of the swapped cap in relation to the tweeter curve in its filter and speaker as suggest Wolf teeth member...
FWIT, Imho and YMMV...of course if I understood it, which may be or not..... s..t it was my dac and the reccording which were both bads

...but on a psychological point of view, I'm feeling myself more powerfull and directive by buying an expensive cap...I'm feeling myself much more clever than buying a good source I can' NOT afford...yeah, often the sad reality, while there is a difference with two high end caps, it's in owner eyes...the gold letters on black or red cap suit... A little off topic, but psychological factor is also rulling as explained in post 3...
Should we sticky this thread to save time, monney and carbon ?