We have a game in hand and a superior goal difference of plus 7 .... how do you justify "they'd almost certainly overtake us on GD"?
Because for them to catch up, they'd have win three games and us lose four if my maths is correct. (Naturally draws can enter the calculation, but let's keep it simple)
Generally speaking three wins will give you at least a +4 GD, while 4 defeats usually gives you a -5 GD at least.
Therefore for them to catch 9 points on us is liable to see them overtake us on GD as well. Before this rout, our GD was looking better than theirs in the event of a draw on points. Hence this win could well be worth four points, and my guess is it would be if we drew on points, as I think this will give them the edge on GD. Even if all wins and defeats were by one one goal margins, highly unlikely as that is, then Arsenal would edge ahead on equal GD, but more goals scored than us.
Naturally, we might reply with a 5 or 6 goal GD win of our own to redress the balance in our favour, but it's unlikely I feel.
Equally, of course Arsenal may come up with another demolition job, again unlikely, but probably more possible for them because in Van Persie they have the best striker in the country, and he is considerably better than any we have. They have already scored more than us, despite having less points. Were they to equal our points, chances are they would have scored quite a few more goals than us by then.