The black and dirty is probably from your PCV valve doing its job (you could replace your PCV valve if it doesn't flow or is stuck).
You can clean your intake plenum, but I doubt it will affect your performance. That trick manifold is not going to help you much unless you have a serious set of cams and other racing parts, as well as the oportunity to use them. Porting and polishing is not the place to invest money in for a stock engine.
Try cleaning your current injectors. Best bang for the buck.
For power, nothing beats a set of headers. Could be more work than you want.
Check your compression. If any of the cylinders is out of spec, fixing that should be your first concern.