@OP are you going to do other reviews and include new sports books on that list if possible?
If the sportsbooks is already doing good on their own terms, I don't think they will pay these review sites to give them positive or high rating. Because even without these review sites, they can already get customers just by offering good services and competitive features. Now, if the bookie wants more positive reviews out there, they can really tap these review sites. But mostly, those top bookies won't as they already earned their players.
By the way, how often is the OP's team updating their list?
Since they're already known to be a good one, no one would notice that they've done that as paid because the other reviews and their reputation is already good enough.