Would you spend £875 for a 3rd party, replacement passive crossover for your SBLs?

Because it's what a guy is asking - apparently not as a one-time sale but as a regular commercial offer - for the item in question, on the famous auction site (the product is apparently made by a known maker of Naim 'upgrades' and is claimed to outperform, or at least equal, an active multiple system).

But weren't those 3rd party replacements makers offering cheap and better alternatives to the too much expensive Naim originals?

What am I missing?