Andy
The inlet of a turbo scroll is convergent. This is before any gases are released across the turbine. Then the scroll continues to convergent path till the end this is to keep the velocity up across the whole of the disc to keep the forces equal across the centre bearing. Aircraft turbo centre bearings are made of aluminium any force that's not centralised will wear the bearing out extremely fast.
Px is distributed equally than as you say there would be no use for the scroll to continually decrease in size.
It dose not matter if it avagas engine Diesel engine or a gas turbine from an small Alison 250 to rb211 they all work on the small priceable velocity 1st the px change across the disc.
Now late model turbos in cars use variable vanes instead of a waste gate. The opening and closing of the vanes changes the velocity onto the disc.
In a gas turbine the same is used on the compressors to stop compressor stall on start up and low speed.
Cheers