Should be fine unless they hit some really quick blocks. GHASH will accept any difficulty so you might as well just set it to 1470.
Some have suggested that raising the diff will help with lowering duplicates at GHash I don't know if there is anything to that though. Others have suggested that the Prisma doesn't handle vardiff that well and think that setting a higher diff can help but I don't know if there is anything to that either. With all that said like CrazyGuy said unless they find a block really quickly it shouldn't hurt.
I asked once already but it got lost in the thread. What is the system load in Minerea actually measuring? Mine are always yellow and didn't know if I should be concerned and would using a wired connection vs wifi lower the system load?
From what I can find it in Minera forum posts appears that is the "load" on the RPi resources.
I currently have 2 Prisma on my RPi using wired connection and my RPi shows Green
http://i.imgur.com/OG3oJIw.png