I did the ridge walk in early December 2012. Starting at Bluff Knoll car park I walked to the cave on First Arrow (4 guys must have been tight!), the next day I completed the walk to Ellen Peak and then dropped down and walked along the fire track back to the retreat. Due to the weather heating up I had to bypass Ellen Peak (that'll give me a reason to go back). There is a rough pad along most of the route, I found that the roughest section was heading around the south side of the cliffs of Pyungoorup Peak, the trick being to follow the pads as close to the cliffs as possible. There was a bit of scrambling up ledges around First Arrow, a rope may come in handy to pack haul. I was lucky that there had been good rain before the trip, I found water just off the track up Isongerup Peak, in the barrel at Third Arrow, and in Woolaganup Creek on the fire track. None of these should be relied on though.
The owners at Stirling Range Retreat are extremely friendly and helpful and dropped me off at Bluff Knoll car park (for a fee), the retreat was for sale when I was there so things may have changed. I used AT Morphetts, and Chapmans notes and they were more than adequate. I wrote up a report at http://hiking.topicwise.com/doc/stirlingridgewalkwa
if you want to see plenty of photo's.