It would help new healers to get more into DPSing for sure but it would completely break any experienced healers for sure. Even something like Stone IV which is 250 single target potency would become some of the best single target damage around. At 2.5 second GCD that is 12 Stone IVs in 30 seconds instead of 6 chaincasted under the old way.

The difference only becomes more pronounced with spells that have cast times longer than the GCD like Holy, 12 holies in 30 seconds is an insane amount of AoE damage. I feel like the meta for AoE fights would just become PLD/RDM healing while WHM Holy Spams with a BLM to manashift them.

Maybe exageration but yeah this is AST levels of OP.