Finish out attic?
If you pull a permit for this work, you will need to do the work per code. In Mass, the reflectix is not considered a substitute for the proper amount of R-value. But reflecting heat is a great idea. Many new roofing products now are designed with a similiar characteristic.
You can now get asphalt shingles, metal roofing, and roof tiles that are treated with a reflective coating that will give a similar performance to a pure white roof (white reflects the most heat of any color), but allow you to use darker colors to better match the look of your house.
If you add living space, you will increase the demand on your AC. If you have a problem with heat in the attic, and are just looking to reduce load in your AC, a better approach would be to better insulate the attic floor, seal off all penetrations through the attic floor, and most importantly is to make sure you have ventilation that is proper for the cubic footage of your attic space.
Also make sure that any bathroom, or dryer vents that go into the attic are not terminating in the attic space, but are extended out to the exterior of the house.