We did that in the past succesfully, it all depends on the quality of the ATB you are charging to your Cat Cracking and how you handle it. In our case we were charging a parafinic ATB wich helps to increase the conversion to gasoline, its crackeability was very good.
We did that on a MW Kellogg design which has a high dense catalyst phase in the riser, which helps to reduce the impact of the higher concarbon fed to the unit. Additionally an good blend of the ATB and VGO is required for better results, otherwise batches of high concarbon material in the feed will cause major upsets in the catalyst circulation and possibly a unit shut down.
The economic driver for this operation was excellent, as our refinery did not have Delayed Coker and part of the ATB was send to bunker fuel production.