Good little shopping list. Well thought out..
A few notes you may want to think about:
1 - Extender - do you NEED 2x? Remember you will lose 2 stops of light, and your lightning fast 2.8 suddenly turns into a 5.6. The 1.4x mk3 is fantastic and you will only lose 1 stop. Obviously you know your needs not me, but worth remembering how much light you lose, I've also found in my personal usage that every 2x i've used has yielded less than ideal results. But i'm not an experienced birder like a lot of these forum members who use them frequently, mine was for photojournalism during some riots where i wanted some distance. I'm sure a lot of people have great results, but even in forum reading i see far more favourable reviews of the 1.4 over the 2x.
2 - Bags - Because you are carrying a fair bit of gear, be careful with backpacks, i find they can be a bit dicky with a lot of weight. I have about a zillion bags and hard cases (I use Vanguard hard cases vs Pelican... same quality and good price).
BUT i find the Lowepro Stealth Reporter series is great. You can quickly access lenses without picking up your bag and dropping things because your clasps werent done up. If it is for work (as you mentioned) stress gets to you and I have a habit of accidentally dropping the most expensive thing in reach when i get stressed.
3 - Primes - 50mm 1.2... an amazing lens. Very interestingly spoken about on forums, those that don't own them tend to slag them off yet those that have them love them. I ALWAYS have it mounted on one of my cameras.
4 - cf/battery - don't listen to the person who questioned the spare battery and the extra card... haha.... I have 6 memory cards, and still ordered more with CR guys post on Sandisk prices dropping, I have 8 batteries and still go close to running flat. When you spend almost every day shooting, or if it's imperative that you don't miss a shot you NEED spare batteries.
CF cards are another big thing too, if you are shooting on RAW a 16gb card doesn't last long. If i was you i'd pick up another while they are cheap... or grab a 32, I picked up 2 x 32gb 90mbs cards at BH
5 - UV filters - i know you have already discussed filters, but have you got uv filters to protect an accidental scratch on your beautiful big 77mm lenses? No point spending the money to put a dirty big scratch on it!
6 - Grip - i love my grip... amazing for shooting portrait, also brilliant for work as when you are on location and you are running out of juice you can throw some AA's in it to get you to the end. Definitely worth the money. Also with video it adds some weight to help you stay steady, as well as being great when shooting portrait.
I reckon since you are holding off on the ultrawide angle, if i was you I'd buy another memory card, a grip (generic with 2 batteries is about 70), and some UV filters....
Hope it helps.... I know how good the feeling is opening up a big box of goodies.
I'm going in to order my 28-300 tomorrow and i'm excited
makes it justice to the quality effort.
