Quite some time back, Joppa suggested that +to hit would help spell as well as melee attacks. However I haven't heard anything to that effect for a long time. VR recently did a serious makeover of stats and their effects. They removed Spell Power (adds to damage) as a stat as well.
Separately, stats that say +X to (whatever) are not to be taken as +X% to (whatever). It could take +hundreds to something to get it maxed, and maxed might well not mean 100% certainty. Currently there are more questions than answers. And things are guaranteed to change.
I hope this has been helpful :)