Speedy Cherche????

I didn't check your calculations, but your concept is correct. People don't talk about it a lot because Cherche is famous not for spd but for atk and brave weapons don't help. That said, I think it's a good idea to build Cherche to be able to quad slow units. Any squishy unit she will easily slay in two hits; high def creatures may require more. And it works especially well for Cherche because she's a flier and it's possible to stack goads on her, so let's say you go +spd as you propose, and you use swift sparrow (NB not L&D if you want to keep her magic soak. -def is ok, but L&D runs counter to your purpose) and manage to stack two goads on her. That's 36 spd without merges, which quads even a lot of dragons, and we haven't discussed ally support, merges, or the weapon refine.

I personally recommend taking the effect refine because panic ploy fodder is very limited. I think there's a very reasonable case to be made for spd. However, if you're merging, I would say >40 spd is probably overkill. By the time you stack that many goads on her, her atk is probably approaching 70 so even stuff like F.Grima with close def might well fall in two hits.

Cherche can't use special fighter because she's not an armour; guard can be good just to make sure somebody doesn't steady breath ignis her. I tend to want desperation, but it's tricky to drop into desperation when so much dies in two hits and the probability of getting hit with steady breath ignis is actually fairly high anytime she needs to quad and doesn't already have desperation active. And she has a lot of HP so even fury takes a few skirmishes to get desperation going.