Tested using Feria with 5 Piety (which would result in a -1 modifier if it were strength) and the spell restored 1-7 HP, instead of the 2-8 it usually does. While less than the normal value, it is only 1 less, whereas having a stat of 16 would, I believe, be 3 points better.

16 Piety test with a Nue using potions yields 21-33, but note that the New has 200% Holy attack, so that would be more like 7-11 without the bonus.

I suspect the 7-11 is really 5-11 in this case. I know that Mark of Ruin does not affect healing, but that it does affect offensive magic. Same with Cruelty, and I belive War Rite is the same way.

Hand of Kindness only affects healing spells that would otherwise heal under 20% of the target's maximum HP.

I am wondering if Feria from a chacter with 4 or less Piety would reach zero healing, and if the game would treat that as a full heal. (Note that this requires reloading level ups until a specific stat goes DOWN, which is not something a rational player would do.)

Also, remember that the SPELLS.csv file can be edited to make spells do fixed damage/healing, and then deleted/moved to go back to the normal values later.