Wait, the Yelp API has a limit of 40 entries, regardless of how finely-tuned you set the geographics. (Which is why I had to switch to the Foursquare API for that purpose)
What is your sample size? If it's large, how did you get past the API limits?
I sent a separate request for each bounding box area and limit to only the top 10 restaurants in the search response. Each bounding box is small enough where I'd say that the top 10 restaurants are a good gauge of how well-reviewed the area is.