Hi Paul,
when the PSP releases the x86 cores from reset, the part of AGESA running on the PSP has initialized DRAM and some other parts of the AMD silicon, but for example the initialization of most high speed serial links like PCIe, USB3 and the display interfaces on APUs are done in the AGESA part that runs on the x86 side that gets processed into the FSP binaries for coreboot. So running without FSP will give you DRAM and low speed interfaces that don't require much initialization like UART and SPI, but you won't have working high speed serial connectivity to external components making the system not very useful for practical applications. Please don't get me wrong, I like what oreboot is aiming for, but even when you only consider the technical side there is still a lot of work to be done.
Regards, Felix