The headband of protection is currently the best helmet by far. Spell Resistance is incredibly potent for protecting against WoF scrolls and also absolutely necessary for any non-Strength build to not instantly fold to a Balagarn's Iron Horn potion in PVP.
It also gives protection from level 1 spells. Notably, this makes the wearer immune to every cantrip. I suggest removing the level 1 spell immunity from this item.
Cantrips are very weak in PVP in general. They are only used by casters that cannot cast spells infinitely. The general consensus is that infinicasters (or casters with spell refund mechanics) are miles ahead of more "traditional" casters. The Headband makes this issue even worse, as infinicasters do not care about using cantrips (since they have access to infinite higher level spells), while traditional casters can easily run out of (offensive) spells to cast.
For the reasons outlined above the headband is already super useful. Even without the spell immunity, it is a must-have on nearly every build that does not have innate SR or uses Rogue's Leathers. It does not also need to randomly punish old-school casters by rendering their only spammable damage source obsolete.