Yes, a prperly designed, seet up, and installed watts is superior to a panhard. However, it takes a fair amount of calculation to ensure that.

A panhard would be an easier install and achieve similar lateral location duties and could, potentially, allow the same adjustment. Either one would require modifications to the spring eye mounting and shackle bushings to allow the springs to pivot otherwise there is high potential for binding as the leafs and lateral device fight for control of the springs. Since the torsional rigidity of the springs resist roll, creating pivoting springs means they may also need to step up in rate slightly to achieve the same roll rate.

I still think for OP's request for slightly firmer handling on a street car, it is easier acheived with in increase in spring rates, better shocks, maybe even a rear sway bar if the roll couple isn't hugely off.