Finishing a song in 5/5 AF3+2 (or 4/5 AF3+2 with Marduk's Tiara for Scherzo) is definitely optimal. Starting to Cast Songs (Precast) in 5/5 AF3+2 or w/e isn't optimal at all. Unless you're stacking Fast Cast/Song Casting -% Atmas in Abyssea, you're gonna be casting slow as hell (Unless Minstrel's is in effect, but even then...) And in Abyssea, you should really aim to not have to depend on those kinds of Atmas since you'd be much more useful with Atmas that gear towards Refresh/Regain to help proc Red/Blue or help support with heals/etc.
Outside of Abyssea... <.< There's no contest that Brd's that care about their job should be aiming to use a Precast set with their buffing songs.
Precast for debuffs isn't really necessary (Perhaps Lullaby though...), and as far I know, Skill/Chr never really "cap" on debuffs. Debuffs have a 95% Cap rate on landing though iirc, so until you hit a 95% debuff rate on whatever, adding more Skill and CHR WILL help with landing debuffs. Nowadays, there shouldn't be too much issue with landing debuffs on most NMs (I hardly use my ToM Ele Acc staves anymore x.x).