both of the watts' on the market now, Fays2 and EVM, are roll center adjustable while most of the panhard bars right now aren't, at least the ones that come with many of the 4 link rear suspension at the moment. the Maier PHB is adjustable but you still have the problem of the leaf springs and teh PHB having different roll center heights in most cases, however the Martz chassis piece that comes with their rear 4 link looks to be an atypical PHB setup and if used with leaf springs looks to have about the same roll center height as the springs.