IIRC at one point they used a machine that alters universal constants to turn a chamber 682 was in to Big Bang conditions and it survived.
In which case, how the hell are they containing it? If regular Earthlings can contain what essentially surmounts to a near-deity, a better question would be is 'would the foundation in the 40k universe stamp out heresy, destroy the xenos and turn daemons into matrix-like batteries in a week?'
They keep it contained with plot acid, which keeps it preoccupied with regenerating instead of escaping.
The Foundation could indeed fix 40k, as they have God.