You could keep increasing that number, or it should also help if you group the files so there are not too many per folder, and not too many top level folders. The more items databound to the grid at once will increase the number of form elements and viewstate.
Also you can remove this attribute from the <pages element in Web.config:
removing that will set the viewstate size to the default which is much larger so that there will be fewer separate hidden elements used for viewstate. ie it doesn't reduce the amount of viewstate but it reduces the number of separate viewstate elements.
The only reason we set it low is because in some older phone browsers it would not load hidden fields if they are very large.
Hope that helps,