Some Google examples:
Restoring it to working condition could take days, if not weeks.
I haven't had a good night's sleep in days, if not weeks, since my girlfriend left me.
Power outages may last for days, if not weeks after Hurricane Ida
These all suggest that "days" is already a pretty long time, and "weeks" is an even longer time. "Weeks" is an even more extreme time, so "sogar" fits.
But in your example
The board‘s decision was accepted with equanimity, if not outright enthusiasm by the staff.
... "equanimity" is not already a positive reaction, it just means they didn't panic or get angry; "outright enthusiasm" is not an even more extreme version of equanimity. So that's unlikely to mean "wenn nicht sogar".
But people do say things like:
She's a good, if not great singer.
That's ambiguous; you could only tell what was meant from the context or the tone of voice.