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.
More fundamentally, the Satoshi proof-of-work is predicated on an assumption that "cost per hash" is relatively comparable, and that solutions are distributed uniformly relative to hashing effort applied. Neither of these are the case, here, which means difficulty can not be "correct" as originally defined. This puts the jobs mechanism at risk in several ways. (Also other bugs for other days, there...)