My use of natural starters has not been frequent enough and intensive enough to be able to speak as authoritatively as others on the forum, but I too have experienced many cases where the naturally leavened dough did not rise that much. Yet, the results were invariably very good, with good crumb volume and crust flavor and texture, especially considering that I was using a standard home oven. Oddly, I have on occasion experienced the same reduction of dough volume using commercial yeast, but in an unorthodox and unconventional way (like using ADY in a nonrehydrated form), and also got a good finished product.
It has also been my experience, as I have noted on the forum several times before, that using a naturally leavened dough seems to lead to a better handling dough, much as you experienced.
With respect to the amount of salt, it is my understanding that it is a common practice in the preparation of authentic Neapolitan doughs that are to be fermented at room temperature to adjust the salt levels to control the rate of fermentation of the dough. This appears to be done seasonally, for example, by reducing the amount of salt in the winter to speed up the rate of fermentation and by increasing the amount in the summer to slow down the rate of fermentation. So, with winter upon us, you might want to reduce the salt to something around 2%, or perhaps even a bit lower. That should also help reduce the degree of saltiness of the finished crust.
In due course, you may want to reduce the amount of your starter once it gets to the point where you can use it reliably. However, I believe that the amount of the starter is related more to issues of dough strength and acid production. That is, once the starter amount reaches preferment levels, acid levels increase and can cause strengthening of the dough by tightening up the protein and creating a gluten matrix with higher elasticity. The higher acid levels and other biochemical byproducts of fermentation can also lead to more intense flavors in the finished crust. By keeping the amount of starter on the low side, the effect of the starter is to effectively serve only as a leavening agent, with a mild flavor profile for the finished crust. At the moment, your use of starter represents about 8% of the formula water. What pizzanapoletana (Marco) recommends is up to 5% of the formula water. I would tend to use the higher end of the range in the winter, and a lesser amount in the summer.