The all dps in the belly is the best strategy out there. If a dps has Unknown agro just step outside.
The main mechanic of the fight is going into his stomach and damaging his stomach walls. Killing 4 of his walls = like 20% HP in the like 30 seconds you're in there and Cerberus has:
2,240,873 exact HP (Cloud of Darkness has about 1,270,000 HP) edit
20% of Cerberus' HP is about 448K HP
448K HP done per second in 30 seconds is about 15,000(!!!)(rounded up from 14,939 DPS) raidwide DPS
Your average WoD instance raidwide DPS for that fight is like 3500-4500
30 seconds is stretching it; the time you're in there is shorter but arbitrarily picking 30 seconds there would have to be a raidwide DPS of 15,000+ outside the belly for it to balance out the damage done by following the mechanic. It's like 10seconds if a caster decides to LB 2 of them.
More DPS = faster instance. Staying outside have 6 healers. considering that boss has like 2 Million + HP your non hardcore alliance raidwide dps is nothing.