If zappers didn't work, I wouldn't think you'd see anyone shelling out $600+ for them, nor would the big name motor tuners bother shipping the 50 lb bricks out to the big races. Granted they work much better on a rebuildable motor, but some have found ways to massage the silvercans on them too. It's not a simple task, and I'm sure many a motor was tossed until finding the sweet spot.
Unfortunately I haven't found the technique yet.