Mousai Gear

Does Mousai +1 set have any utility? Or is it just a placeholder until more solid effect/duration gear is acquired? The price of the pieces on AH lead me to believe there may be use for them.

It's the best for each specific song they boost.
Technically not really the best in the case of Head, potency is the same provided by Empy+3 (through Set bonus) but with Mousai+1 you at least get a boost in duration.

specific song macro pieces is all I have ever used it for

The primary reason I ask is because the only one I actually attempted to use was for Threnody, and my Dark Threnody on Aminon was weaker/shorter because of it.

Then it has to be a user error or an issue in your lua. The Threnody +1/2 extends the duration and potency, not shortens it.

Are you certain you didn't use a Dark Threnody II with SV Nitro initially, and then overwrote it with a SV non-Nitro Threnody? SV songs can overwrite each other of the same type (even if the song you're overwriting has a longer duration), so if you did that, your second cast of Threnody would have a shorter duration than the first cast. I have seen a couple Bards do this on Aminon after they sings songs; They scramble to get their Aminon songs on and then are scared that they might have missed Threnody with Nitro, so they sing it twice and end up overwriting the long duration version with the shorter version. This causes the Threnody to fall off very quickly before Aminon dies, and then you can't put it back on without a SV reset, which is usually a wipe. Very common BRD brainfart.

As others have said, the Mousai +1 pieces all remain the "best" for their specific song, at least in song+ value. They're just somewhat more niche songs (Etude, Threnody, Carol, Minne, Mambo) than the ones that get their similar boost from Empy gear (Mad Min March Ballad Scherzo) or AF gear (Lullaby, Paeon). Can easily see the effect of 3/5 of the Mousai songs with /checkparam, and for Carols you can check your elemental resist on the equipment menu to verify.

For Threnody, it either lands or it doesn't - there's no variance in the duration based on a partial resist or anything like that. Threnody+ gear increases both the potency of the resistance down effect, AND the song duration. Base duration is 60 seconds, plus 6 additional seconds for each Song+1 (so, Mousai+1 head adds 12 seconds to the duration).

Another possibility for what might have happened to Galkapryme here:
Did your Threnody set happen to remove any other pieces of song duration gear? Like, maybe you swapped out an empy body, AF feet, Inyanga legs? That would make a difference, losing any of those pieces would be a reduction greater than the +12 sec gained from Mousai+1.

If that's not it, sounds like you might have overwritten a Threnody with a shorter one like Buukki said, or maybe just an issue with a macro/LUA and gear is not properly swapping in for midcast. Song+ gear does need to be equipped at midcast, so be sure it's in there and not just in for precast and then removed before the song goes off.