oki i see you dont know how old mantra work.
You forgot that mantra have 10 s cd.
we can do now true hps:
1) you charge mantra -> cast take ~ 2.5s that heal you for 3275 hp
2) you use 2 chargs -> it take ~2s 1640 hp * 2 = 3280 hp
3) then cd of the skill -> 10s
Now hps -> you get (3275 + 1640*2 ) hp in ~20s that give you 327,75 hps
And you can add next 1k hp * 2 for Restorative Mantras and 2 conditions removed from Mender's Purity if you will go for it
Last rework of mantra was for mesmer HUGE NERF.
Ofc yes in PvE they will be weeker because whan you charge mantra you can't AA, but for PvP and WvW it is BIG buff.