^ I have not seen data on this issue of why city driving tends to build up more deposits. I don't know the exact drive cycle they were using in their tests, and how much idling or cold starting it included. On the other hand, we don't know much about the E85 you have available to you either. Perhaps a lot of idling or cold starting increases the problem.
Another thing to keep in mind is that this GM engine was an actual flex fuel engine with special ECU maps specifically for E85. So that introduces another factor in the discussion--tuning. It's difficult to find published info on this--all the research is being done on direct injection now, which runs very different fuel systems.
Is there anything stopping you from putting a tank of gasoline in there once in a while?