If money really is no object, check out Nunatuk. I could only dream of affording their stuff, so I haven't tried it but it looks excellent. They use very high fill down, and I think they'll let you substitute even higher if the bags aren't expensive enough.
Of the bags they sell, the Raku and the Alpinist look closest to your specs, although warmer. The sub-alpinist is the weight you want, but only a half zip.
I like the Golite 1+ season bag, but its only a half zip and rated to 40 degrees, so not quite what you're looking for--although you could check it out.
Take a look at the Mountain Hardwear Ultralamina. It's a little overdone with features so its heavier than necessary, but a decent bag.
Check out the review section of the BPL forums and see if there are some other sleeping bags on the market. Most BPL folks are into quilts, but there's still a lot of mummy bag buzz.
Also, do a search for similar threads because there's tons of talk around about sleeping bags.