Max,
I have steam bent wood the size in your question. My wood was not sealed ,and anytime I have steamed wood with some finish on it the steam has taken the finish off , or partially so, and the steam still penetrated.
On your 30" radius question, if you are meaning either a full circle or half circle....you would need a mechanical jig and a live steam table. If you are talking about a partial bend (like the limb shape of a sweeping recurve) then that is very doable.
The major enemy of steam bending is the time (temp loss) between steam and bending and how long it takes to complete the bend. A steam table pre-steams, and continues to steam throughout the bend. Most bowyers steam the wood, remove it from the steam and then move it to the jig, caul etc. And that timing (even seconds) is what can make or break the results.
Maybe that will help you some.
rich