Impact is huge and not just because of cutting-down the prepress time. Of course it depends upon the specifics of your company.
If we talk about the cutting down the time spent on prepress. It depends upon how much of manual and time consuming work your pre-press operators do. How do you preflight? File corrections? Reports? Layouts? Print marks? Export-import, conversion and......tasks? What about the MIS? And so on... Your workflow should be analyzed to understand what stages can be automated and what stages can be better organized. Only after that you will have some approximate forecast.
Error reduction: What is the medium cost of a prepress error? Do you have any? Kinds of errors? Is AE capable to avoid such kinds of errors?
Again, lots of questions to answer...
From personal experience: The impact of AE is huge not just because of time saving and error reduction... AE lets a company to analytically organize the prepress process, make it much more traceable and transparent - this is where the real profit is.
So:
PROs:
+Time saving. Approximate impact - you need to find the answers to the bunch of questions
+ Error reduction. Again, answers to the questions

+ Re-designed and normalized

workflow (that is the real gold!)
CONs:
- Work to do on implementation
If you need any details - write me to PM