My take on this, having heard a few posh kettle lead tests is one of three things happens, depending on circumstance:

(a) There is no significant difference to the stock lead

(b) There is a subtle difference, but too small to reliably identify which lead is which

(c) There is a noticeable difference, but it's not clear - to me at least - that it's an improvement. It's just different. It may sound like an improvement in the moment (often the music gets fractionally louder, I've noticed, and we all know how that can make any of us think something's better). Different <> Better. In all honesty, I've never heard a posh kettle lead make things unambiguously better. YMMV.

-- Ian