In writings about high savings in Japan, the pension system is actually one of the most commonly cited explanations. It falters on two points. First, although Japanese pension benefits used to be quite low, today they are pretty good, at least for employees. Figured in exchange rates they are the highest in the world, but even at PPP they are generally better than most places outside Sweden etc. Most employees get most of their retirement income as an anuity, not in a lump sum.
One might say that people don't realize their pensions will be adequate and so high savings behavior is a lag from when benefits were poor. But Prof. Seike of Keio and others have studied saving rates of households of government employees (who have had quite good benefits for a long time) versus those of other households with not-so-good future benefits. There is a statistically significant difference, indicating that anticipated pensions makes some difference, but it is extremely small. For all practical purposes the savings behavior of Japanese households is not much influenced by the level of anticipated pension income.