Adapters are often NOT a good idea. Quite simply, and adapter will increase the back pressure. Most fans used in computers produce fairly low perssure output, and they often just don't work well with the increased back pressure.
What's more, if you take an 80mm fan and a 92mm fan of about the same noise, the 92mm fan will usually have a higher CFM rating, but a LOWER pressure rating! This can result in LESS airflow from the 92mm fan with and adapter than from the 800mm without and adapter.