AGS (or whatever you call the southern part of the forces) run into majority of Soviet forces (Soviets though main thrust will be sounth of Pripyat, not north as it was). If there is no AGC (split between AGN and AGS) AGS has to punch itself through Soviet forces. AGC can't assist in closing Kiev pocket. German lines resemble <, exposing looooooooooooooooooong flanks to any counter attack. AGS is able to push Soviets back however as they advance they have to peel forces to guard their flank, as there is no AGC to be there. Overall I think their advance stops somewhere along OTL lines with addition of looooooooooooooooooooong, exposed flank.
Now, if you want to play with objectives, combine AGC and AGN and have Moscow as primary and Leningrad as secondary goals