Speaking of which, besides performance how would you guys say direct oGL calls might improve a slick game, and are there any other resources (info) focusing only on the Slick-compatible/useful parts?
For a start: a wider range of blend modes, sorting sprites based on the depth buffer (i.e. hardware), masking with the stencil buffer, collision detection done on GPU, dynamic shadows/lighting, explosion effects, blurring/depth of field, normal mapping, etc. These are all possible using lower level OpenGL/GLSL alongside Slick.
Unfortunately, there aren't many resources yet. But Slick is not doing a whole lot that would interfere with low-level GL calls, so pretty much anything can be "compatible." You just need to take a few precautions before you start using GL.