The reason it's cut off on the edges like that is because the shader can only draw within the rendered area. If you want to get effects like that, you're better off compositing a screen together to include elements that could exist behind the text. Since the parameters are tooled so high, there isn't much reason to make it adhere directly to the text displayable area.
Also I have a sprite/entity version that's in the works. Vimi's been working on something that's got me working towards another pack released, and these type of effects for characters would be included.