Pretty much every thread talking about doing broken things with Persistent spells use Divine metamagic to get around the +6 levels thing. That tells me that the problem isn't with Persistent spell.
However, I do have a non-broken problem with Persistent spell: that it simply replaces the duration with "24 hours" at a cost of +6 levels. IMO, the cost should differ based on the original duration, from maybe +1 for an original duration measured in hours, to maybe +7 or +8 for one measured in rounds. Something like the way you change the pricing of continuous magic items based on spell duration.