IMO the quaiffe is a better pure track day diff than the giken. If you want to autoX/drift/drag/track the giken might be more appropriate and versatile.
I run RPF1 285's square with no fitment issues, some guys will use a small rear spacer.
Eibach/Stillen front with OEM rear is a solid setup on the OEM dampers/springs. I think anymore would overload the front too much.
2cents