Sounds a bit hokey to me. Tho the review on that page seems to say otherwise. $35 to improve a J-bass? It's a sight cheaper than the part-ex on a decent bass...lol

Why not give it a try? It might drag your headstock down a bit, and I've a feeling it might take a bit of jiggling it about to find the sweet spot. But then, so many pleasures in life are found that way...
Adding weight to the headstock is an old trick to improve sustain. Sometimes it works, sometimes it don't. Fender basses should be the last that needed anything like this.
Before wasting $ 35,- on this, you can easily test if it works by clamping, tie wrapping or gaffertaping some heavy object to your headstock. Let an unbiased and blinded listener do the judgement and then hear for yourself by letting someone else play some notes while you listen without looking or otherwise knowing what is demonstrated. My bets are that neither will guess it right more often than chance predicts, but if you do get a measurable result, go for it.
Another thing to do would be to play softer and turn your amp louder.