u can see diseases doing less dmg on other target on main (there is calculation from your current AP on other targets), but main target keep same value from time when u used IT+PS, so if u use proc everything (scale+dbw etc with maximum AP) your dots will do so much more dmg when u keep them up with pestilence, this increased my dps atleast 1k dps on single target
about WP it seems it hasnt internal cd so its just about luck