>> the SoC could handle at least twice the amount

> IIRC it was advertized to be available in configurations up to 1 GiB RAM.

This would have been dishonest advertising then, considering this:

"256-MByte addressing range per Chip Select (Two CS lines available)"
https://www.nxp.com/docs/en/reference-manual/MPC5200BUM.pdf (p. 15)

"256 MB addressing range per CS, two CS available"
https://www.nxp.com/docs/en/data-sheet/MPC5200BDS.pdf (p. 1)

"256 Mbyte addressing range"
https://www.nxp.com/docs/en/product-brief/MPC5200BPB.pdf (p. 3)

So the maximum could have been either 256 MiB or 512 MiB.