I have no experience with that yarn, but the only reason I thought about the stretch is that I made a sweater with Debbie Bliss Merino Aran and didn't really swatch. I started the sleeves, measured, and said 'good enough.'
When I washed it, though, it grew tremendously (it's a superwash wool.) I dried it and rewashed it, but it didn't get significantly smaller. I wore it a couple of times but it really was enormous, so I ended up frogging it.