I'm not sure about what is lost never comes back. I know that has been applied to hearing, but that's a different type of nerve that cannot regenerate.
I suppose if the facial nerve is "stretched" by a large tumor, it can't be magically "unstretched," so deficits caused by that may not improve, but at least it won't be stretched further; that makes sense to me.
If a smaller tumor in the IAC is causing compression or diverting blood flow, that seems to me like maybe it could be expected to improve some over time after the offender is removed, assuming there is no major additional damage to the nerve during surgery.
Anyway, these are total layman's speculations and I wouldn't worry too much about the %ages. You still have a lot of useful function and no visible deficit, so consider it a matter of academic interest only! If you come out with what you went in with, or even close, you'll still be just fine, and any amount the nerve is "irked" during surgery, it can come back from.