I did a little testing on the mastery.
My tooltip says "Increases the potency of your direct healing spells by up to
31%, based on [...]".
I have 3895 Spell Power and used no buffs (no earthliving, no totems, nothing). I have 604 mastery rating (which are 4.42 mastery). I am level 84.
I casted a few heals with Riptide which really has almost no variance in its upper or lower bounds as Philondra has already mentioned. I noted down the percentage of HP i currently have and the amount that the direct portion healed (the HP percantages are rounded to whole numbers so there might be some slight inaccuracy).
I entered the values into a spreadsheet and the results were the following:
So yes, it is obviously linear and shows the results that could be expected.