Actually, it WAS NECESSARY to go through immigration, collect bags, enter landside, take a lift to departure floor and then check in bags again. Not too painful as it is in the same terminal but necessary. When we reached PVG, there was actually a CX member of staff waiting and helping all transitting passengers to do above. Hence, there is no better way.
I emailed numerous depts at Air China and phoned them and was told my baggage would go straight to Tokyo and not need collected in Shanghai. BUT now one of them has said I will need to... getting the truth seems impossible. I'm going to assume we do need to collect them and go through the above.