Their was a post about this awhile back that pretty much concluded that they do help, probably not as much as advertised on a vehicle with good grounds to begin with, but they cant really hurt. Honestly, for the money you can make your own using any high grade car audio ground cable and your pick of gold plated ends. You could add one good ground from the battery to the frame and another good ground from that point to the engine block on each side near the plugs somewhere. And if their is a localized spot in the engine bay where a number of things ground at, it couldnt hurt to add one there. You can even daisy chain them from one point to another if that would be easier.
Where does the coils ground to? I'm thinking if those ground to the block somewhere, route a nice 4 or 8 gauge (8awg being smaller with the core being roughly the diameter of a pencil) wire from the battery to the frame, daisy chain that to the points where the coils ground and assuming that isnt somewhere on the heads, daisy chain that to each head near the plugs if possable (consider getting unshielded stainless ground straps so you dont melt anything from the heat of the headers). The plugs and coils are where the juice is used in the engine anyhow. The only other major use of juice is the systems inside the vehicle and all the vehicles lights which wouldnt improve performance.