With AmEx MR you can borrow against future earning and if you haven't caught up a year later you have to pay for the points you borrowed. The rate is somewhere around 2.5 cents/point, which isn't horribly unreasonable for buying points.

I'd say that $400 worth of points really isn't too big a deal. Do you have plans to replenish the account or were you planning on taking the points and closing the account? As long as you replenish the account in a reasonable amount of time you'll probably be OK.