I went through the whole process. Based on my experience, a law firm can't help you much on EB1/NIW, since the most important evidences are reference letters. Lawyer knows nothing about your field/achievement. He/she only submits all your materials you collect. He/she may help a little bit on the cover letter, whcih can't also be done by yourself if you have some sample letters (In my case, I completed everything and handed over to my lawyer.) However the application submitted from a law firm may look better in the eye of immigration officer than the application filed by yourself. I am not sure whether it's really true or not, but the statistics supports the arguement. Since an immigration office only spends very short time (about 15-20 minutes?) to go through your application, every positive impression counts.
My advice: Find a less expensive a law firm without bad reputation. Now effiency also matters due to 10/1 deadline.
Hope it helps. Good luck!