There are a couple of low cost abilities that you can stack.
The problem is that you are assuming that runed effects take effect prior to passive effects.
The 15 might happen first, which means that having both of them attached only saves 2.5 hate compared to just having the passive.
imho, grenades at the moment suck. They start off with less damage then chakram and then the only ability that adds a real benefit to anything is the stun grenade. Even poison only maxes out as an un-runed grenade iff the mob stands still in the poison.