Hi Joerg,
The warranty on Phase backs is very worthwhile; in my experience (UK) dealers work extremely hard to provide backup, even to the point of driving backs to users in extreme circumstances.
P30 files are around 32Mb in IIQ large (which is lossless), IIQ small uses less space, but is only visually lossless.
Personally, I would agree with Roland; I tried V system, but found H lenses to be superior. If you buy a V series back, Phase will change it out for another camera fit (H series for instance), if you decided to change camera within the first year.
Best,
Nick.