Like promised test showed that it indeed works and increases caster levels and dps of druid.
Print screens will provide proof but i will explain a couple simple things first at each screenie.
First this is split i used:
2nd in this screenie i used action spellpower boost because cores of sorc give 30 spellpower for elec to be as close as possible, best would had been if i would use a 48 elec weapon but since i did this on thelanis due to no character slot on cannith the difference in spellpower is around 12 spellpower in sorc and no sorc, so action boost was used to come as close to possible to the same spellpower with the resources i had.
This is the 2nd print screen which clearly shows the increased damage mainly gained from casting call light at a higher caster level, also note no action boost and same season.
The difference in spellpower would not result in a such a big increase in damage, if calculated it is clear indeed how sorc cores increase druid spell caster level and thus damage.
I have no idea tho why the caster level doesnt show in first pscreen, i used in sucesion first the spell then the sla and tested multiple times with reset and came to the same conclusion.
You can go ahead and plan onward what you think is better, but sorc split would clearly give you more dps in chosen spell element.
IF you wish to splash do so, if no no, but due to test it would be optimal if you splashed.
Now how much you want to splash depends on your personal preferences.
You can stay pure, idc, but i wanted to give solid proof that you would gain a bit more dps with splash, dont hate me since im a test freak and love helping people improve builds if i see space for improvement.
Again dont take this personal, if you were offended i need to say my attempt was not to do so, but to provide help on how to improve your build.
Cheers