Yup based on my prior experience with WM Alpinelite I think I will go that route again. 1200MM is quit good considering how thin the fabric is (12d). I will lose some breathability not using Microfiber; however, I never really thougtht it was an issue as I ran that bag for 4 years. from June - end of October.
I have a 10+ year old WM Micrfiber bag and poured a glass of water on the exterior shell and let it sit for a bit. When I poured it off, there was no water penetration I could discern.