Nobody keen to have a go?? Must be voodoo?
I think it is because you are asking many question in one (possibly without recognising), mixing subjectivity and objectivity from an apparent position if ignorance and no-one cares to take on such a task.
To break it down...
The reason why one may wish in Op-Amp decoupling to tie the positive rail and negative rail together through a cap are down to several issues.
Op-Amp's have of course a "ground pin" (and yes, most of them have ONE ground pin - it is the one with less PSRR) and stability often improves if we link the Supply pin to the ground pin through the shortest possible loop. One may then take extra measures to minimise the loop impedance between the Op-Amp ground pin and the actual ground.
What sort of capacitor we use there would of course depends strictly on the actual application and the Op-Amp used.
As for X7R, they have very low ESR and their ELS is pretty much down to the physical length, sadly their capacitance is very much modulated by voltage (and to a lesser degree current) and so they may be okay for low end products or where we deliberately want to add some H2 dominant distortion at low frequencies (I have used them like this explicitly to achieve this).
What will be the subjective effect - again there is simple quick answer, again it depends on the specifics. Adding some H2 dominant, low order HD at low frequencies often "warms up" the sound.
As said, way too many questions in all of this including some where the answer highly depends on context.