It is not reduced by anything, including, but not limited to, tactics and (de)buffs.
Some WP's started using this a fairly strong way to heal.
You can improve it with the SM -5s CD buff even more, and get insane 4560 hps on your group with focused mind.
While i don't mind that it actually heals for the amount of HP buffed, it did not heal on live.
But right now it starts being a problem, since there is no counterplay, you can use it without a single point of willpower and any heal reducing tactic you want to.
As example of the sheer amount of healing I present you this sc statistic:

With at least 450k overheal by just two WP (ab)using this.
Healing more than half of your healing with the same Buff is definitely not intended, in my opinion, but maybe there is a reason I don't see.
This might at least get some more attention than Bugtracker entries.