What is a 15.5 driver?

Probably what is meant by that is a driver with 15.5 degrees of loft. Typical drivers are 10-11 degrees. Big hitters might go a little lower, while slow swingers can benefit from more loft, maybe 12 or 13 degrees. 15.5 is pretty extreme; that kind of loft is usually seen on 3 woods. If someone is touting a 15.5 degree driver it probably has a very large head, of the size that is usually seen on a driver, and a flatter lie angle to accommodate a driver-sized shaft of 44 inches or so.