Ironically small capacitors are cheap enough, but that does not stop the manufacturer's cost cutting department from forcing both the size and quality down a notch from what the designer would have liked when the unit is put into production. By default I check the calculations for the 'as built' design and adjust values accordingly, whilst selecting the appropriate capacitor type for the task in hand. I don't see that being much different to loudspeaker crossovers. The gains may not be as big perhaps as a typical well considered crossover upgrade, but there frequently are significant gains IME.
Designers are after all human; they have blindspots, favourites, prejudices, sometimes smugness, and like the rest of us make mistakes too. I have a long list of design faults I have found over the years in consumer and professional audio equipment, and price or company reputation is no barrier to faulty design.
Where possible I have always tried to communicate those findings back to the designer. Often they have been very grateful, but every now and then there's one who couldn't give a rats and isn't going to talk to someone who challenges their (self perceived) godliness. For example there's a certain high-end phono stage in which I discovered an earthing design fault that caused noise in some setups. When raised with the designer he fobbed me off, yet the Mk II version had the modifications I had devised and sent to him. Another designer was very grateful when I discovered the reason for an extremely low level sizzle noise in a high-end valve preamplifier caused by a trimpot used for setting bias. The designer didn't know he had exceeded the current rating of the trimpot and had just been replacing the trimpots under warranty as needed.