You dont need brushes to make a good sig. Theres lots of tuts on Game Renders for filter only, default, smudge. Stuff like that is what you can also do. Just letting you know. They also got brushes on GR.
Render looks fimiliar lol and as for the sig itself its got alot of empty space I would suggest trying smaller dimensions try a width of 300 or around that its much easier to manage the sig that was and try to make the render bigger. Also try brushing for a little then try moving to defualts its more satisfying to see what you can do without brushes.