If the clutch is being done it makes sense to have the diff fittede while the box is out.
Baffled sumps arent essential but nice if its seeing more track action
I'd defiantly fit a diff while the box is out. I had no dout in my mine that I was going to have one fitted when my was out. They're amazing and not the expensive if you're having a new clutch/flywheel fitted at the same time. The levels of grip are are much higher than I imagined they would be and understeer and wheel spin are virtually none existent.
I'm very surprised that they are not more widely used on our cars? Especially as cars like the Renault sport Megans and RS Focus have utilised them so well. Plus brought about the steady demise of the Impreza and EVO. I would never by a hot hatch again with out one fitted.
They're fantastic everyone should have one.