Hello evnhali,
I wish that I had a definitive answer for you on the question of “How bad is the performance impact of Scene Capture 2D?” The short answer is that it depends on what you intend on using it for. If you are going to be making a large scene with a lot of static meshes, particle effects and post process material, the cost of using scene capture 2D will probably be too high to consider an option, however in a clean, simple project it may be a feasible solution. The best way to test is to set it up in your scene, play in editor and decide if you are comfortable with the tradeoff.
Thanks,