They do work and the 390 needs it in my opinion. Re-gripping from a high throttle position to brake can be dangerous. and if you don't re-grip and brake with some throttle on it obviously reduces braking performance. I've experience both of these scenarios.
The above option is good if you can find the right piece for the job. I did mine by gluing a couple bits of pvc pipe into the existing slot. The cable has to pull around these then line up into the stock slit afterwards.