The 8th SPE was supposed to be re-used for the PS2 softemu that was used for the alter PS2 classic tiles on PSN, but I don't think it ever materialised.
It can be reactivated on some of the later models using a jailbreak, but it's only useful for Linux/OtherOS.
Yeah it was odd they used the GDDR3 for the RSX, considering it had access to the FlexIO bus and could read from its XDR RAM,
at a reduced rate. I don't think the Toshiba GPU would have been any more of an improvement, considering how limited the PS2's eDRAM was for textures, and how they planned on using the same technique for the PS3.