Yeah I think dagger sinX deserve a buff, but it should be on the loki's dagger since that one was intended for PVP use. Hel's is more of a breaker weapon.
The sp increase can be pretty useful in breaking when every pot counts and you don't want to waste your pot just to recover some sp for EDP. With the increase you can squeeze another EDP in between every use of pots.
Since the daggers are all lvl4 weapons now there is a slight increase in the damage they deal, but I think for sinX to be really versatile all skills should be put into consideration. Skills like VenomSplasher can't even be used properly, because of the HP% restriction. In the current environment it's pretty much impossible to cast the skill on anyone, because they pot up so fast that you won't ever get the chance to use the skill.