Is it worth promoting a -Spd +Res Corrin?

Not with -Spd. She wants to keep her Spd intact.

Mine actually is -Spd +Def, unpromoted and she does carry her weight but obviously is not as good as she could be. You should probably get one without a Spd bane and her Res is really too low to properly tank without quite heavy SI investment.

You don't want a bane in Atk/Spd/Def for F!Corrin.
Spd is her worst bane, since thats what defined her(before her children came). I have a neutral one and she is doing a lot of work, but I wouldn't suggest promoting a unit with bad IVs if she's 3-4* avaible.