Mat, UltraCam won't stretch or creep, even when as few as 6 strands are used. Have heard some good thins about Astro, but have not tried it. With flemish strings made of FF material you can get stretch when there are too many twists. Kind of like a coiled spring. I experimented much with 6 to 8 strand strings. Very quiet provided the bow is in tune. In the end I have found that 9 or 10 strands is better. I really see no difference in performance, getting the center serving right on an ultra skinny string is a pain, and 6 strands just isn't as tolerant to all the things that can break strings particularly in hunting situations. The trick is to keep the number of twists at the optimal number as I stated in my first post. The reason I started experimenting with strings that have a lower than traditional count was to get a quieter shooting bow. Picking up a little extra energy isn't bad either.
Keep in mind that I am talking about flemish strings. I don't make endless loop strings and I know there are different issues with building them with lower strand counts. A 10 strand endless string would have 5 in the loops. Not sure if that is safe. I really have little experience with them.