My bike came to me with the furbur fix already done. It's speed wobble is pretty inconsistent. Sometimes you can cruise at 135-140 no problem. Other times by 110 or so it's all over the place, so it doesn't seem to be that effective.
You can buy the spanner, but a flat screwdriver and hammer work just fine. Set the tightness using the "bounce test", Morley has a video of how to do it somewhere.
The furbur fix may seem to work if the bearings are fine but just misadjusted(too loose). If the bearings are shot, it might help a bit, but you're covering up the problem instead of solving it. It works because you are adjusting the bearing tension, not from swapping the washer. Since you have new bearings, I'd just set them properly and go with it.