....Anyhow, to try and help.
I used a MOD issue arctic bag on last years course and found it just a little lacking.
( Sleeping in one set of wool base layer )
Things improved for me when I remembered to have my last hot drink a little nearer bed time and to do my star jumps just before getting into my sack to get my temp up.
Now as a gortex bivi bag is a must, your down 5 season bag will be a great choice and will be protected.
( Just re-read your post, I see it is a 4/5 season bag, I guess you know how warm it runs for you tho ! ?
I will be EITHER teaming up my arctic issue bag with a Snugpac Elite 3 or My Snugpack 12 ( Merlin? ) with the Elite 3.
I will check both setups for bulk, warmth and weight and then make a choice.
Hope this is of some use.
My best.
Chris.