The idea is to minimize the volume in the hull that coukd be filled with water. So anything of significant volume that you can stuff in there will increase safety.
By the way, carrying filled drybags on the deck is not a good idea. Not pnly does that negatively affect stability, but junk on the deck makes self rescue very difficult.
A dry bag filled with gear defeats the purpose if you want to use it to replace a float bag. While it will reduce the amount of water in the boat, it will not provide as much floatation because of the weight of the gear. This may affect the ability to self rescue.
An empty dry bag will work as a float bag, although it will not fill the space as well as a properly designed float bag.