What I disagree with specifically here is where the word "quality" is used here, and the logic that follows from that premise. I've Googled around to figure out what makes Sprague Vitamin Q Paper in Oil Capacitors of a higher quality than caps of other dielectrics, and I can only find info suggesting they are high quality in relation to other PIO caps with regard to their construction, but nothing to suggest they are higher quality with respect to sound, or in relation to ceramic or other types of caps.
I would agree that, as an experiment, $4 is cheap, but you're not conducting an experiment when you do something as a matter of routine.
And when you talk about "small percentage of the overall cost", suck-cost-fallacy does enter the discussion, because you're saying that the size of a prior investment serves as the justification for further investment into uncertain outcomes.
When you post something on a forum, you should expect that people will read what you write and respond with their own contrasting opinion and views. This is the only forum where I ever seen a plurality go to such great lengths to shape the discourse, and enforce a particular kind of propriety when it comes to how people respond, and what they respond to. Apparently in this forum, it's OK to call someone "captain obvious", but it's not OK to challenge someone's opinion. It's somewhat bizarre, to be honest.
Ultimately, this is substantive, on topic conversation. The merits of spending extra money on guitar components is very worthwhile topic and appropriate for a message board such as this.