One thing which I can say for sure is that the difficulty calculation will need to be made entirely per work order job, as some programs will execute at 200k evals/sec/cpu and some will be 2k evals/sec/cpu and (as things stand right now) some might be 0.0002 evals/sec/cpu.
I tend agree with this assessment that the POW should be per work id.
My understanding is that the owner of the slow eval/sec work is supposed to provide larger POW payouts to entice miners to mine their work; however, I could see this having the opposite affect because of what HunterMinerCrafter wrote. Regardless of how much the POW payout is of an extremely slow eval/sec work item is, with only 10 POW payouts per block (for all work ids combined) there would be a good chance that even if you found a POW result for the slow eval/sec work you wouldn't get paid as the fast eval/sec work is going to get evaluated 10x, 100x, etc times more often than this slow work and would likely consume most of the 10 POW payouts.
The other issue I see with this disparity between slow & fast work is that it seems very likely that the fast work will end up paying out all their POW rewards, but the slow difficult work may only pay out a small fraction of it due to this disparity between slow and fast work. It would be great if we could figure out a way that the POW rewards get paid if there are miners working on that work id.