Thanks for the replies. We just got back from our trip; the SW cape presented fewer difficulties than we anticipated and only presented a couple of extra challenges over the South Coast track which we walked on the way out.
We found the SW cape easily navigable and the track quite visible for most of the circuit. Indeed, we felt the route was better delineated in the 'untracked' areas (esp. between murgab and horseshoe) mainly because it didn't dip unnecessarily into overgrown gullies or follow tall coastal heath (often we'd be pushing through vegetation on the southern parts of the circuit while only metres to the side there were clear buttongrass plains or spurs). Overall, however, we encountered only light-medium scrub at worst and never such dense vegetation that we were slowed to frustrating levels- our slowest pace at any point probably only dipped to 2kph. I had mainly been concerned about facing the sort of scrub that would slow us to 1-200 metres p/h; having encountered this before it does not make for enjoyable walking.
We carried a simple Etrex on this walk and while it was fun to use it wasn't indispensable. Crossing the SW cape range, from Wilson's Bight to Window Pane Bay, however, turned out to be a real trial. We were met with very poor weather along the ridges- horizontal rain (some sleet), thick cloud, and heavy winds. The GPS was really useful here because the cloud cover reduced our visibility to about 20m for most of the time and conditions were too poor to bother scouting for the track every time it disappeared. It took us 10 hours to walk that traverse (by comparison we did the Ironbounds in less than 5 hours) and only stopped once for a 10 minute lunch (too cold otherwise). In fact it was so cold up there we each needed to wear a thick polartec baselayer under our shells- gloves, glasses (stinging hard-driven sleet), fleece caps too. Because we expected worse weather throughout the trip, however, we were safe, dry and well equipped- I've never been so thankful to have invested in quality hardshells though. This exposed section of the route is a good reason to have the track marked as being for experienced walkers.
There were some funny moments too though; we reached horseshoe inlet at a fairly high tide (3 creek swims) and the waters covered a lot of bog areas. As a result we'd sometimes be wading along quite happily before suddenly plunging unsuspectingly into a bog up to our mid thighs with the water level higher again. At least the water washed all the mud off quickly.
I'll post some photos after I've compressed some of them