No, you're not getting it, we already get 40% leech in pvp with that talent right now and it is absolutely worthless. Having a 10% out of demon form will make absolutely no difference at all, this is pvp where we have to comply with pvp templates, you do absolutely no where near the damage you do in pvp compared to pve and instanced pvp.
To give you an idea, your health pool is just shy of 6m, in general pve my dps is anywhere from 1m to 2m, i heal for 100% of that with leech, its ridiculously good healing which takes me to full and basically feel immortal.
In pvp my dps is drastically reduced where i do no where near that kind of dps (i would have to go ingame and check accurate numbers but i am sure we are closer to 200- 250k in pvp) and i only heal for 40% of that IF i take soul rending, i am still trying to heal a near 6m health pool. You don't have relics, you don't have set bonuses, you don't have enchants or gems,you don't have trinket procs etc etc. You just do basic damage.
Leech healing is non existant in pvp in legion and even if they removed the 0.x multiplier our leech gives us in bfa it will still make no difference to our health bars.