The bit that would make me wonder about 399 is the memory. Not sure how RAM prices are these days or what it means for motherboard complexity/size.
Otherwise I'd say it has a very good chance of hitting that number. Yes, it's a bigger GPU than Pro, but it's a year down the road on the other side also...surely that brings some cost savings?
Depending on that, the memory factor..I wouldn't rule out 399 yet. MS wouldn't make money on it, but it would be a good point to reach if they can. I think Scorpio is a much more attractive box vs the base Xbox One than PS4 Pro is to PS4 (which is as much about the relative value of the base boxes than the upgrades, to be fair to PS4 Pro), but 399 would still be a safer choice to aim for than 499 or even 449.