I am not familiar with the SnS But I am guessing the more the better
I am in the process of moving and thus have no studio. but the place we are moving out of I had an attic studio with 2 X12 rafters and 12 inches of fiberglass batting with no sheeting on the rafters (other that heavy white visqueen) and I thought the diffusion of the rafters worked fairly well.
the peak was about 7' 6" but of course the walls (ie roof) was angled which helped also .
Somewhat like what you are doing.
Here is the google sketchup I sent to the speaker manufacture when I ordered my monitors
This is what it actually looked like
I hope to be building a home studio from the ground up at our new property.
here is a great resource for anyone doing a home studio.
http://www.johnlsayers.com/phpBB2/