Think of AAS as an accelerator. They can accelerate you up to and past your genetic limit. That theoretical limit is a little flexible though, depending on diet, suppliments, sleep, etc. etc.
Imagine it would take your 15 years of lifting to hit your genetic limit. AAS, when properly used may be able to condense years 10-15 into a one or two year period instead.
Now if you use AAS to blast *beyond* your genetic limit, once off gear you'll regress right back to somewhere within your limit.