GAF stormguard for low pitch roofs

I am putting a roof on our studio apartment that has an area of 2.4/12 pitch and have been advised that an Ice and water shield would be a good idea in that area. I was hoping to use GAF stormguard because from what I understand it is an ice and water shield but when I looked at the box it did not have directions for use on low pitch roofs and said not to use it full coverage unless it is under a metal roof. Looking around on the web I found this

http://www.gaf.com/Roofing/Resident...er_Watch__XT__StormGuard____Un-218-960-v2.pdf

Reading this I get the idea that the no full coverage issue is due to the lack of being able to pass moisture through the stormguard.

Has anyone used GAF stormguard for low pitch roofs? Our roof ventilation is great and there are almost no obstructions so I assume I am OK on the moisture issue? I live in Northern California and we have ice or snow here so maybe I will be OK with just double felt?

Please help me decide what to do!!!! I have to start putting my shingles on Thursday so it is time to make a decision