The part of the population that cares about Voidwatch certainly should be looking at maxing out their idle set. I mean, that IS what we are talking about right? I can't imagine what else you could be bringing 2 PLD to. Voidwatch is something you shouldn't really be devoting massive amounts of time to unless you're essentially capped on Abyssea because Abyssea is so much more rewarding for the amount of time spent.
I love how you say "conveniently" as if I intentionally didn't mention Mandau or Mythic because I intended to mislead or misinform. Aegis and Ochain don't provide a special spell or ability because they already have unique and unmatched traits. It should be readily apparent to anybody how awesome and unique these shields are. As for Mandau and Mythic, what's your point? We're talking about Ghorn. I think you're trying to argue that BRD with it's B- dagger skill being able to use Mandau is somehow the same as a melee with A/A- skill and actual melee JA/gear?
It's pretty clear from availability of 91-95 spells that SE does not care whether everybody can get every spell. Your suggestion isn't horrible. Daurdabla already has its niche as the special instrument that lets you have a 3rd song. Ghorn has always been the king of potency (well, in theory). Its unique trait could be additional potency above and beyond all instruments. But Ghorn shared its +2 status with a dozen instruments at 75 and +3 instruments were introduced at the same time as Ghorn 80, 85, and 90 trials. So it seems to me that SE does not value keeping Ghorn's potency ahead of the pack.
Which brings us to where we are. I think SE will give us +4 on the Ghorn99. Even if by some miracle SE does not introduce +4 instruments, the benefit of +4 over +3 is very small due to skill and haste caps. Even if you add an additional +25% (as you mention in your example), you are still limited by caps. Haste caps. Scherzo caps. Refresh becomes redundant.
The whole reason we're even talking about a Ghorn-specific song is because of this post by Camate:
http://forum.square-enix.com/ffxi/th...l=1#post168159
That quote is the reason people speculate over and clamor for a relic song. Because SE hinted at it. As I've said multiple times before, I'm not convinced that SE will put Ghorn99's potency ahead of the pack. And even if it did, this would not be a sufficient anyway. It needs something on beyond that.