I think it would have been very hard for the Axis to make any further offensives in the East come 1943 that would have had any significant strategic outcome to the war. The men and material could certainly have been better used in fighting a defensive retreat, but in the end the outcome would have been the same, just in a different year. The only time that Germany could have possibly beaten the Soviet Union was in 1941, and perhaps very early 1942, after that the clock was ticking to the endgame.
The BBC have a nice article:
http://www.bbc.co.uk/news/magazine-23137492