How long were you using oxycodone? What route (snort, swallow, inject?) of ingestion? How long have you been on Sub (3 days) and why are you not talking to your Doctor?
So many things can be going on, that this is something you need to talk to your doctor about. No, your habit is not too large for Sub. Furthermore, when you are in your early withdrawal from full agonists, things are going to suck no matter how much Sub you take. 300+ oxycodone a day is a pretty decent habit.
My suggestion, call your doctor immediately. He might be able to prescribe some other meds, like clonodine. I was given klonopin my first few days.