Belly Drum Arceus would be quite interesting.
Firstly, Belly Drum Arceus would fare poorly against hyper offense, where currently offensive teams are the most vulnerable against Ekiller (it fares rather poorly against stall). In hyper offense, almost all Pokemon are usually able to deal significant damage to it, and between SR, Belly Drum and a powerful attack Ekiller would die having accomplished nothing at all most of the time. Really, hyper offense would be more threatened by Ekiller, since it really relies on its good bulk (which is immensely compromised by SR+Belly Drum) and lack of weaknesses to set up, and +2 Silk Scarf Arceus can OHKO most Pokemon hyper offense likes to use (and often survive a hit if it fails to OHKO). Belly Drum Arceus would simply fail to find suitable set up opportunities, for risk of dying or random Pokemon carrying phazing moves (Giratina-O, Dialga, Latios, Latias among other good hyper offense Pokemon can all fit a phazing move, and losing 50%+entry hazard damage to be phazed makes for a useless Pokemon). It would really almost need Wobbuffet to set it up against hyper offense (although if it ever did it would easily 6-0).
Against stall on the other hand, Belly Drum Arceus could be quite devastating, as stall often uses Pokemon that cannot significantly threaten or phaze it, allowing it to come in, set up, and do enormous damage. If Belly Drum Arceus were to become a common sight, stall would be forced to have a way to hamper its sweep (burn, toxic, a powerful attack, phazing) on every Pokemon (since you essentially get 1 free turn on it as it sets up and that is the only turn you really get to do anything against it).
Due to the enormous disadvantage of Belly Drum (losing 50% health), combined with the prevalence of entry hazards in the metagame and the fact that a heavily weakened Arceus still needs to give the opponent a free turn while it sets up, I don't see it really breaking the metagame. If anything, it would make the already dominant playstyle, offense, even more superior and prevalent, which would be unfortunate. Really though, the whole point of Ekiller Arceus is its great bulk in addition to its ability to be very threatening after only a turn of set up, and Belly Drum loses it more bulk than it can really afford.
Also, calculations are cool and all, but Life Orb may be the single dumbest (commonly used) item Belly Drum Arceus could run. Leftovers/sitrus berry are the only items which make sense, and calculations are only useful so long as they're realistic.