Whilst I agree that we can't say it for sure, because we know knothing about Hiruzen's prime and merely saw a rather disappointing perfomance of an aged one, I don't think that he was the strongest Hokage. Even if he could perfom anything he showed so far ten times better in his prime, that still doesn't make him the strongest Hokage. It's most unlikely that he possessed a Kekkei Genkai which he just decided to hide during the fight against Orochimaru, and just because Enma is based on a japanese story/legend of the monkey king (which is a pretty good discovery) it doesn't mean that Enma can/could do anything that monkey king could.
Hiruzen was considered the god of Shinobi because people actually believed in his abilities. Unlike Hashirama, whose strength must've been so immense that people considered it a fairy tail - thus not believing it was actually real/true.
Nevertheless, the proof extracted from both manga and data-book is actually valid, even though not a 100%. As far as I know, even in the data-books it's said that 'he was praised/called the god of Shinobi', not that he actually IS the god of Shinobi. Even in the data-books, it's still a hype, not a fact. And a hype is only true until it's proven wrong: even if it was stated in the data-books. The best example would be Tobi: In Sha no Sho, the latest data-book, it was said more than once that he IS Madara. On Madara's page there even is a picture of Tobi. But as we all know, this turned out to be a lie. It was what Kishimoto wanted us to believe at that moment. That doesn't completely disprove what was said in previous data-books, and doesn't completely mean that Ksihi only wanted us to believe that Hiruzen was the strongest Hokage back then, but it shows, that we have to be careful with content, even if it's provided in a data-book.