VANCOUVER, British Columbia - Mighty Cahill finally lived up to the promise he showed when he won a maiden special weight race by 11 lengths in his debut as a 2-year-old last September. Under a heady ride by apprentice Mario Gutierrez, Mighty Cahill came from slightly off the pace to win the Burnaby Breeders' Cup by a widening 4 3/4 lengths last Saturday and stamped himself as a serious contender for the Grade 2, $250,000 B.C. Derby Sept. 24.