Andreas and me are actually working on improvements on how QGraphicsView handles the caching. Of course we have some test-applications to validate our work but Plasma provide us a pretty good real use case. Here is a screenshot that we took on Plasma using a small trick to see what is cached and what is not cached.
In red, NoCache mode. Green or blue show items that are cached (green is DeviceCoordinate, blue is ItemCoordinateCache).
It seems that the caching in Plasma can be improved. 😀