One of the greatest aspects to a mountain bike is the ability to run fat tires. Fat tires offer a very plush ride compared to skinny tires. On an 8 mile commute I'll take plush 2" slicks over harsh 23-25 cm tires any day of the week. On a rigid bike, your tires are the suspension. There is a VAST amount of difference between 2" wide tires and narrow road tires. The large gains in comfort and flexibility, would largely outweigh any small losses in speed on a reasonably short 8 mile commute.