Uggh, I entered a weird situation in FEH.
I got a Grima for merging, but its stats were perfect.
+Attack -Speed
The only time a merge's IVs matter is if they're better than the other one you already have.
If your current Grima's IVs are also perfect - you merge him. Doesn't matter what his stats are. In he goes.
If your current Grima's stats are
not perfect, you feed your current Grima to the perfect one instead. All his prior merges, inherited skills, and the like will carry over, and you'll be working with perfect IVs from then on.
The weird question comes when a unit has several different "perfect" IV setups - but M!Grima ain't one of those, making it a pretty easy call here.