I'd love to see real pictures of that fire extinguisher, and what it looks like under the seat. Probably don't want to see the price.
It would go well with the OEM euro warning triangle I have on the trunk lid.
As for overkill, I've never seen a requirement for a fire extinguisher for any AutoX or HPDE. (Don't know about TT.) Is it on the required list for event you are considering?