My listening test revealed no SQ change at all - so long as the data is arriving is still bit perfect. But with optical when it fails, it is fairly easy to spot bit failures. Of course, YMMV, and I guess if it's about to fail, you would hear an improvement with the odd bit error improvement. But I have not been able to hear a difference in my setup using plastic or glass.
Why would that be? Optical actually does not have bad jitter performance; but what it does do is have uneven rise and fall times. But my digital SPDIF receiver actually measures uneven rise and fall times, then uses that measurement to compensate to extract the data correctly. And as regards jitter - the DPLL completely removes jitter from the incoming stream - I can add 2uS worth of jitter, and see absolutely nothing coming out from Mojo with measurements - and Mojo's FFT noise floor is at -170 dB. So optical typically has 2nS of jitter, so that is a thousand times lower than a level that is still not detectable, even when I can resolve -170 dB.... So there is no technical explanation why it would make a difference. So if you do hear a difference, it is either because it is not bit perfect and has data errors (almost impossible with 44.1 though), or your suffering from a placebo (it looks nicer/costs more/must be better).
Getting to your last point - the DAC has a big impact on this; most DAC's are very sensitive to jitter as they use analogue PLL techniques and they can't eliminate the jitter problems. So optical cables may have a SQ difference with other DAC's.
Rob