Also, as @Inferno Vulpix can probably elaborate on better than I, we're technically wasting long term XP when we don't have only prime doing the studying, so it may be worth stopping anyways now that we're on diminishing returns notes.
The main thrust here is that once we got our hands on the notes we were already entitled to all the XP within them. Whether it's now or later, sooner or later all that XP will wind up within Hazou's character sheet. Meanwhile, every single training block we spend FOOMing is bonus XP we can't get back if we pass it up. If we divert 10 XP worth of clones to note-reading (and then FOOM optimally otherwise), we will wind up exactly 10 XP behind the Hazou who FOOMs optimally without any clone-note-reading.
Which isn't to say that there's no value in having clones read notes, we've seen with our own eyes how much value a Sealing push like that can accomplish. But returns diminish over time. Let's suppose diverting 10 XP worth of clones speeds up our notes-reading by 40 XP: that's very worth it in the short term. But what if we've hit the worse notes? When we hit the 2 XP/day notes the payoff is halved, but the expense remains the same.
Moreover, since the benefits of clone-note-reading only manifests in terms of
short-term payoff (every clone spent reading notes is intrinsically net-negative on our XP totals, remember), the value of the tradeoff is dependent on our short-term situation. Higher Sealing gives us two immediate boons: we can research more difficult projects, and we can research projects faster. The former goal was very poignant when we started doing this, but now that we're bottlenecked by calligraphy we can't get any more value out of it in the short term. The latter goal remains useful, but marginal for all but the most difficult of seals (especially given that we physically cannot attempt seals at the limit of our Sealing ability due to our calligraphy bottleneck).
In short, not only are we starting to run into the lower XP/day notes with strictly worse payoffs than the initial notes, we've already extracted the vast majority of the value we could possibly gain from this maneuver. Our Sealing is, in the short-term, functionally maxed out, and any benefits to it result in very marginal improvements to our research capabilities at best. Meanwhile the cost of each clone diverted from FOOM remains constant, continually shaving slivers of XP off of our medium and long-term XP totals. Overall it was a very fruitful maneuver but I think it's time to stop.
[X] Hazou should only use Prime to study Sealing notes.