Whether or not the consulting firm is credible has nothing to do with it: simply put, it's been shown time and time again that studies like these are full of manure. As spisska notes, a large part of it is that money that would be spent elsewhere in the city on gameday is spent at the stadium, taking away from other businesses: there's not a boon in spending, the spending is still there, just in a different part of town.

I'd advise you to read the section in Soccernomics on stadiums. Kuper discusses how building new stadiums rarely provides any sort of major economic benefit to the area. I'd quote actual passages to you but i left my kindle at someone's house.