How does this work exactly?
"Heightened (+1) The amount of healing or damage increases by 1d8, or by 2d8 if you’re using the 1- or 2-action version to heal the living."
Does that mean that a heightened heal spell at level 3 heals the same amount as a heightened heal spell at level 15? Or is it an extra 1d8/2d8 for every spell level above 1st?
It's for every level. The "+1" means increase the spell level by 1 to get this heighten effect. If you increase it multiple times, you get a multiplied heighten effect.
Much appreciated. The rules as written, at least in the version of the playtest document I have, weren't very clear.