Some of the most respected researchers in deaf ed say that there don't exist valid studies supporting the claim that Deaf of Deaf outperform Deaf of Hearing in reading comprehension or other indicators of literacy. In "Psychological development of deaf children" Marschark reviewed the conflicting studies conducted on deaf of deaf vs. deaf of hearing and conducted an assessment of the results, and an answer. There are studies that "show" that deaf of hearing outperform, and studies that "show" that deaf of deaf outperform. But, he summarized by saying that the findings were inconclusive, there didn't seem to be an advantage either way.
What's critical, though, is that 50% of 18YO Deaf & HOH students read below the 4th grade level (equiv. to a 9YO), compared with 1% of their hearing peers. Think about this -- these adults with low literacy are not good reading models for their children -- deaf or hearing. This cycle is what affects literacy -- it's so very complex, but in general, deaf children who read better had their hearing losses diagnosed earlier, had early access to fluent language (usually via sign language), and were exposed to English.
If you are interested in the subject, take a look at Marschark's Raising and educating a deaf child: a comprehensive guide to the choices, controversies, and decisions faced by parents and educators -- 2nd edition, the Learning to Read and Write section.