Much like Lynn said... and I'd also agree with her advice to go for a bag.
Further, if you tend to sleep cold, the Western Ultralite is 7 ounces more than the Summerlite, almost all of which is added down to a shell the same size. The Ultralite does also have a draft collar (insulated tube around your neck, cinch if cold, leave loose if not). Draft collar helps if you tend to be cold. Regarding a draft tube, backing the zipper, there's no insulation where a zipper gets sewn in, so a good bag will have a good, beefy insulated tube behind the zipper...
Lastly, I'd count on your bag for your warmth... not the aggregate possible warmth of adding a bivy. Bivy's fine if you're tarping it and want some extra weather protection, but not really an insulation thing.