My understanding is that Parnelli Jones used Watt's link to try to get a better balance for the rear end's lateral movement. As you probably already know, the panhard bard is only set up to compensate for taking turns in one direction and not both.
I did a lot of reading and investigating on this myself a few years back. Like most things, the answers seemed to differ depending on the source: some people swore that the Watt's link setup changed rear end stability when turning, and some said it was no better than a panhard. It also depends on what kind of Watt's link system. I read that you want one that is not anchored to the rear differential... probably worth some deeper investigation.
I have a hard time thinking that this doesn't work being that Parnelli used it and it was offered on Saleen Mustangs. If you're like me, though, you will want to see/experience proof! I decided against it because for the price, I could invest in other things that matter more to me since I do not do road or course racing.
Also worth investigating how this would affect other characteristics. In engineering, you find that you create a secondary problem every time you resolve an initial problem.