Hi Johnny,
there is nothing wrong with +/- 16V in the original API500 specs.
and it is doable to do 24V modules with it (DC/DC converters).
the reason we have the +/- 24V in the GDIY 51X format is DIY

DC/DC converters are not totally easy (if you want them right),
it is much more easier just taking the +24V directly for Neve / Neumann / Telefunken "clones",
and for sure +/- 24V plus the +/- 16V allows more current draw for each module

as far as I know the main problem / concern / bashing comes from some API500 PSUs not giving the specified current for each module.
and 130mA @ 16V wont result in 130mA @ 24V using a DC/DC converter ...