If you upgrade any components, not just on a bass, to ones of 'top spec' you surely expect an improvement? Otherwise why waste money buying 'better' pick ups, or top quality strings over cheap ones?
Why pay for quality tyres on your car? They're just black round things right?
A very important point made by the OP is that if you're changing pick ups, usually at considerable expense, you may well not get the improvement you expect if your wiring + all the components in the signal path, (not just pots), capacitor, wiring & soldering are not up to a high standard. You can't deny that.