I just notice Vigor drops 6.5Kg in CC from Turnbull. Lost by 8L in Turnbull and Viewed drop only 2Kg where Viewed lost by 3L.

That's hard question. I think it would depend a lot on conditions and the size of the horse. A horse won't necessarily run faster with less weight on say a good track over1000 -1200m. Above that distance, in a truly run race rather than a jog and sprint, weight starts to play a more important role. The wetter the track the larger the difference it should make too. You can't really put a weight = a certain difference though.

had a quick look on RaceCensus:

Horses down in weight by 5 kg or more in a Caulfield Cup since 2000.
Only 1 winner from 48 possibles, at $6.80

Filtering out all horses at $20 or more, we are left with 1 success from 21