[asshanson]

Probably not an O2 sensor if both are reading lean. Your fuel trims are maxed out on the positive side right, it can't add any more?

I would guess this is an air metering problem if your fuel pressure at the rail is good. Maybe an air leak pre-turbo but after the air sensor? Have you gone into boost? If it goes super rich under boost you may have a massive air leak somewhere, letting in extra air under vacuum and letting out air under boost.