I'm not concerned but I do think it's an interesting tactic and worth discussing. Assuming they're going into the red at that price, one wonders a) how long they can keep it up and b) whether it was a smart move in the long run.
If they're going to spend a lot of money to get the user base, now is the best time to do it. Good sales, good word of mouth etc, it won't harm them.
The long game for them will be to try and get a lead in the US which... well I didn't really think a sizeable lead was really that possible but after this month, if they can win December (on the basis of closing the gap further, not actually "winning") and then keeping the price at $349, then maybe it is a possibility for showing demand and interest in the system.
I would be very interested to see how going to $349 and bundling FH2 with SO or Halo MCC would do for them in the "quieter months" of the year which I think they should definitely do. A bundle of their own games would presumably reduces their losses (not paying a third party publisher for the games) and is very competitive whatever Sony offer up for The Order and Bloodborne which will be good months for them [Sony] to win.
MS have to be below PS4 in US and offer good deals, if they do that then they can get the user base there to sell big numbers on their exclusives and 3rd party titles, which will help reduce losses and as the systems costs come down can help them to eventually not be leaking so much in bundles etc.
If MS go hard in the US, I don't think Sony will go out of their way to match them on price if they're actually making money on the PS4 and the offers they have