Phase/Mamiya 645 DF Focussing screen

Does the Mamiya FD 402 focus magnifier make a difference? Will it significantly help with accurate manual focusing? It sells around $200.

Looks similar to the magnifier for the RZ67, so I'll comment on that. Manual focusing with a prism is always a compromise compared to a waist level finder because of the decrease in magnification (0.81X vs 2.9X) and brightness. The magnifier for the RZ prism, I find, compensates for focusing accuracy but means you can't see the whole frame at once.