Duritz,
Your point about raw sectional times being unreliable (because of the differing pace of races) is valid. However it is possible to make adjustments e.g. If you determine the normal (standard, par ,whatever you call it) percentage of the final 600 sectional to the total race time for each track and distance you can then compare this standard with the actual race in question. i.e. if the final 600 proportion in a race is lower than standard (i.e. a sit-and- sprint) you can penalise the sectional times of all horses in that race.
The trick of course is coming up with accurate standards for both 600 times and the percentage of total race time. This is a pain - particularly with new tracks such as Hillside.