They both sound correct to me. When I looked in "A Practical English Grammar, 4th ed," (333), I found a similar construction:
"as" can mean "though/although" but only in the combination ADJECTIVE + AS + SUBJECT + TO BE/ TO SEEM/ TO APPEAR Tired as he was[,] he offered to marry her. = Though he was tired[,] he offered to marry her. Strong as he was, he couldn't lift it.
Even though your grammar does not follow the combination that they described, I understood your sentences to mean the same thing: Hard as he worked/As hard as he worked, he still went out of business. = Though he worked hard, he went out of business.
Again, they both sound correct to me, but I would say that the "as ... as" construction is more common. And when I Googled "hard as he worked," most of the entries that seemed to mean "though/although" (in a similar construction) had "as" before "hard as he worked": for example,
But hard as he worked to improve as a sculptor and engraver, it was the order Durgin's received from a certain Dwight Davis for a silver trophy that would prove to be Rowland Rhodes's enduring masterpiece. http://www.daviscup.com/about/history/silvercup.asp
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