Hi Plantpot, that mod is really aimed at the 650 bikes. The wire between the rectifier and battery is really too thin for the task. This causes a voltage drop between the power that comes out of the rectifier and what the battery actually gets. This has the knock on effect of not supplying enough power to the cdi and a weaker than intended spark being produced at the plug, thus causing running issues. It also plays mayhem with the ecu on new models.
Look in the 650 section and you will soon find a post about it.
I have done this to my 650 and it worked a treat, however, I'm not sure it was ever an issue on the 125 and 250 models.
It probably wouldn't hurt to do it, but you may not notice any improvement.