Check here:
http://www.flyertalk.com/forum/ameri...s-dl-only.html
And be sure to read some of the later posts, as Amex changed their stated rules in the past few months. Recent reports are that Mileage Booster still reimburses, but the door is closing (or may have fully closed) on gift cards.
Also, Economy Comfort and preferred seats is absolutely reimbursable, as they are considered seat assignment fees. You're in the same cabin, so it is not deemed an upgrade. Also, I burned through my entire $200 this year on DL Economy Comfort and preferred seat fees.
For your remainder, you could use it on SkyClub day passes if that's of interest to you. Even if it's not, I believe you can buy physical day passes at actual SkyClubs, and they make nice gifts to friends/family who might be taking trips soon (just check expiration dates).