A bit of a late response, I've only joined this subreddit recently and I've been looking around to see if people here are actually interested in the theoretical side of history. I'd say that Edward Said's work was part of a larger body of works which greatly influenced modern day historiography, though in my own opinion they had both positive and negative effects. While I do think that his theory and methods were far too polarizing, his thesis exposed a problem worth adressing and his influence is undeniable.

The positive effects were many and they are all intertwined. I'll try to keep it clean and simple. Works like that of Edward Said pointed out, rightfully so, that contemporary historiography was still burdened by a "Western" bias. This made a lot of sense considering that writing history on a scientific level was mostly built on a tradition of great Western authors. Historians were forced to acknowledge this bias and in turn they had to reevaluate many of their methods and tools. Because of this historiography also received the attention it so direly needed. These days historiography is a very important part of any history education and that is partially owed to works like that of Edward Said. Another important consequence is how many historians from different parts of the world started experimenting with different tools and methods which fitted their subjects better. While this sometimes yielded unusable results, it also led to some interesting and original works and promoted history as an academic discipline in previously underrepresented regions.

A third notable consequence was ofcourse the emergence of global history, which truly invigorated the field of history. In an effort to lessen the gap between different regions of the world, historians have been trying -for a few decades now- to unify grand theories to better suit regional differences. Even when doing more specific research, academics are more sensitive to this notion now. As I said, I do believe that it had some negative consequences aswell. While it's very beneficial for any field to reevaluate and critizice it's foundations, in this case it sometimes led to a witch-hunt. In the heat of the very racially and culturally sensitive debate, many historians were far too polarizing in their opinions and blind to their own personal bias. That includes Edward Said in my opinion. Because of this, many viable methods or concepts were heavily scrutinized because they were reminiscent of a "Western view". This was partially reversed when historians remembered that they themselves defined their methods and concepts, not the other way around.

So in conclusion, Edward Said's work had many positive consequences within historiography. That being said, it was part of a "movement" within historiography and there is a larger body of works responsible for these consequences. So it's rather difficult to gauge the effects of a single work.

Said's book is mostly just an extension of subalternism, which is the study of people who have been either politically or culturally dominated by another power. As for it's influence in Middle Eastern studies, I'd say it has influenced modern historiography a fair amount, but that same influence could have also come from historians trying to distance themselves from portraying history as tales of Great Men (or Great Powers in Said's case).