The rule change was made for engine capacity; from 990cc to 800cc. The MotoGP bosses believe that speeds are getting too high (all but two or three riders reaching 200 MPH on the Italian Grand Prix straight), thus decided to limit engine power. Some believe that GP bike horsepower will be down from 250+ to somewhere between 200-225. Consequently, the bikes will probably be harder to ride - more abrupt power delivery with the 800cc as compared to the smooth power bands of the 990cc engines - but faster lap times are still possible. I believe the Kawasaki GP team has seen their riders go as much as eight MPH faster through certain corners with the new, lighter, more compact 800cc bikes. 2007 will definitely be a season to watch!
Hope I answered your question.
Time to defend the Title NICKY!