We tried something similar to the saga you proposed to "stress-test" Realms of Power: the Infernal when it came out; though perhaps what we did was grittier still.
In our saga there was a single, fundamental difference compared to canon Ars Magica. There was only one supernatural Realm active on Earth: the Infernal. The Divine existed, but you could reach it only when you died, if you had not succumbed to temptation and mortal sin first. Magic and Faerie did not exist: they were just masks for the Infernal. The Gift was really the False Gift. Miracles were delusions at best, Infernal tampering at worst.
The saga lasted six sessions (it was tightly scripted and meant to be short), and was ultimately very, very gloom and doom, more than any White Wolf game we played so far We liked it immensely as a diversion, but I think it would have become rather boring beyond, say, twenty sessions or so. The current Ars Magica setting offers a much richer palette with which one can paint stories.