As you suggest, a new PoS block means a PoW miner has to abandon the work it has done, and start on a fresh block. There would only be no effect if they were on separate chains. Both types of mining are competing for the next spot in the same chain.
PoW - 3 min spacing? - 480/day?
PoS - 5 min spacing - 288/day
Total: 788/day
I'm not a stats person so I don't fully understand the probabilities involved with block time prediction, but I guess that means you would derate the estimation by the 480/768 ratio, so an estimated 8 hours for pure PoW becomes 12.8 hours for PoS/PoW.