IDT - I'm quite happy with the products I have. I know what you are saying. For a few quid more you can have a huge gain. And to be honest that's why I (and most people I would imagine) buy these accesories. Better cables, better speaker stands, better av furniture, all to squeeze that extra few percent out of our much loved kit.
I'm no expert either but I know that if a product came out that 95% of people agreed gave a 20% improvement for an additional 1% in cost then I would probably buy it, I'll be honest I do struggle to tell the differences between one quality cable over another in the same price bracket. But I do acknowledge that a £100 cable will make more of a difference than a £20 one (depending on the quality of kit its attached to).
However I also do think its much harder to see a difference in the difference an accessory makes and the difference say a different better quality component makes ( a better cd player for example).