Erm, no you don't..
Firstly bus width has absolutely no connection with the ability to perform stencil buffering. Second Flipper's memory bus to main memory is 64bit not 48bit. Finally, am I right in thinking that you've just tried to claim that GC could not render shadows?....
Ok,yes,I remember for that N pattent about the pre-calculated shadow ,and yes,you can cast shadow even multitexturing(use the fram buffer as a texutre),but for a fast and nice shadow you need a stencil buffer.Of course there is workaround,but the majority of the developers are used to the stencil buffer.
No,it is not about the flipper-main memory bus , but about the bus to the flipper internal frame buffer .