Cat5 does gigabit some of the time, Cat5e does gigabit most of the time, Cat6 does gigabit all the time, assuming you have a gigabit router. Are you IT needs local (on your own network) or are you accessing remote resources through your internet connection? Cat5 is adequate for just about everything except an actual office building.
A full HD stream is 5 megabits per second. Are you streaming 200 HDTV shows at the same time (or the equivalent)? Is 8 seconds too long to wait for a gigabyte of data to download? Sounds to me like the installer is trying to screw over people that don't know that cat5 is perfectly fine for almost everyone and almost every application. Most people don't even have gigabit internet to take advantage of the network speeds available.