Fast Cast Set For Locked Weapon?

I'm picking up stage 5 Opashoro shortly, made it for SMN but I'm trying to figure out if I can use it effectively on SCH as well.

In terms of nukes I'm thinking I'll just swap Empy body to Agwu body to recover the magic burst damage I lose from Bunzi's Rod, and it should be an upgrade. I should be able to do that whether I utilize aftermath or not.

Where I'm struggling is the fast cast set if I decide to lock weapon and try to get AM3 to really get the benefit of the weapon.

My current FC set has 69% without weapon/sub:
ItemSet 395447

I'd like to keep both grimoire pieces in (the head & feet) but I was recently made aware that grimoire casting time is capped at 15% (25% total including the 10% native bonus) while the pieces give you 12% and 13% so in practice one piece isn't much different from two. The second piece just gets you an extra 2-3%.

First question: Can anyone confirm if this is true? Is there truly not much value in running both of the grimoire pieces? I trust the source of the info, but I haven't been able to find any tests.

Second question: Even if I drop one of the two, it doesn't get me 11% so I'm still under the FC cap. There are several headpieces with 10% so that gets me 1% short. It also drops the larger of the two grimoire pieces, the head is 13% while the feet which I'd be keeping are only 12%. Does anyone have any better ideas for this? Hopefully I'm overlooking some juicy fast cast pieces somewhere.

(Please don't suggest /RDM. My main use for SCH right now is Sortie which I go /DRK for.)

HQ Pinga Body is +2% over Zendik, IIRC.

Can augment Merlinic head/feet with FC+7 using fern to get +15/12 respectively if you swap one of them.

You can get +7% FC on head or feet merlinic to replace a grimoire piece

Amalric Coif +1 has 11%, is relatively painless to obtain, and doesn't involve throwing rocks at Oseem.

Oh I somehow overlooked Amalric head. I already carry that so it's not even inventory+1. Good call, thanks!

Still interested if anybody knows of a way to do it while keeping both the relic head & AF feet, but I'm thinking it's not possible.

Depends if you're in Sheol: Gaol but um...sub RDM?

As an aside: wondering how Light Arts/Dark Arts interact with FC, additively? If so, unless you're casting spells opposite of your arts, you're already at the cap with the set and recommendations?

Head/Feet: Relic/AF
Body: 15%
Hands: 9%
Legs: 13%
Neck: 5%
Ear: 4%
Ear2: 2%
Ring: 4%
Ring2: 2%
Ammo: 2%
Waist: 5%
Back: 10%
71% plus 10% in arts.

It's completely separate. It effectively lets you go beyond 80%. WHM/SCH with Light Arts and capped fast cast for example will be at 82% just from the native Light Arts bonus.

Edit: This also includes any grimoire bonuses like the "Grimoire: Spellcasting time -13%" on relic head. That means the light/dark arts reduction goes from 10% to 23%. It does not mean you need 13% less fast cast to hit the cap. This makes using at least one of the two grimoire spellcasting pieces extremely powerful. If you aren't doing it, I highly recommend it.

If on SCH and using both of the grimoire pieces you're apparently capped at 25% grimoire cast speed reduction. So if you're capped at 80% fast cast as well, this would take you another quarter of the way putting you at 85%.

Since I'm dropping the head for Amalric when I lock my weapon I'll be at 22% grimoire cast instead of capped. That should put me at 84.4%.

Anecdotally, I tried this in Sortie last night and didn't even notice a change in cast speed. Still casting faster than everyone else :) It's possible there's some rounding happening and it literally didn't even change at all.