Surely a good headset will be sufficient?
To some extent, yes. But the sound waves also reaches your inner ear through your skull, and might cause a hearing damage over time. Flying up to 7-8 hours a day in 90-100 dB wouldnt be concidered healthy anyway, but a helmet in my opinion, wil reduce the dB.
What were Norsk using prior to the change?
Mainly David Clarke headsets.