I think the HK upgrade is worth €420 as you do get an extra amp and speakers and agree disconnecting the centre channel is a logical start point, but it certainly isn't worth the list price.
I compared the BAVsound/Focal/ETON upgrades and settled on ETON purely on OEM fittings, availability plus getting them for
a good price on Amazon.
Recently there have been a few bad reviews of the BAVsound upgrade due to poor build quality - magnets falling off speakers - so didn't fancy having to return them to America.