I do some calcs:
Detailed Result:
252 SpAtk Life Orb Starmie Ice Beam vs 252 HP/252 SpDef Trevenant: 46.35% - 54.69%
Possible HP Damage: 178, 180, 182, 184, 186, 188, 190, 192, 194, 196, 200, 202, 204, 206, 208, 210
2-3 hits to KO (with Leftovers)
252 SpAtk Expert Belt Starmie Ice Beam vs 252 HP/252 SpDef Trevenat: 42.45% - 50.52%
Possible HP Damage: 163, 165, 168, 170, 172, 172, 175, 177, 180, 182, 182, 184, 187, 189, 192, 194
3 hits to KO (with Leftovers)
Done with the new base power of Ice Beam (90)
Edit: Starmie nature is Timid and Trevenant Careful in both cases