Ok, there seems to be some miss information in here.
HQ staves are estimated to have 20-30 m.acc, it is a static number, not a percentage increase (only the nuke damage is a percentage increase).
I am no BRD, but assuming their m.acc works in a similar way to 'normal' magic.
When you below 10 dSTAT Alky > HQ
If you are above 10 dSTAT HQ > Alky
But the difference is minimal either way.
I believe SE made them to be equal most of the time and that's pretty much what the maths shows
Alky has the advantage of 10 stat on it, which makes it great for potency based magic (mainly Para and Slow), elegy isn't potency modified (I don't think any brd song is), but CHR does increase accuracy.
A pretty good bard will sit around ~110 CHR in their debuff gear, so should be over 10 dSTAT (except on really big nastys) therefore HQ is likely to be better.
Alkys biggest advantage is it can be used for any element, therefore you can get way with carrying only 1 staff (well 4 if you inc Terra, Pluto and Apollo [idle, resting, curing]). But considering BRD only needs Terra and Apollos for debuffs then you haven't really gained anything.