Cool bike Gummee! !
I saw a friend tonight who is a racer. He just won a 40+ class regional series in both the Hare Scrambles and Enduro. I asked him about mountain biking and skills carry over. I'll see if I can get him to come here and comment.
His feelings were that mountain biking definitely helped him skill wise in addition to the fitness for racing. He said that during his winning racing season, he increased the amount of mountain biking he was doing. He said that while riding a mountain bike and trying to push fast, he became better at picking the fastest lines because on a mountain bike momentum is everything. He said he definitely felt like this made him faster on the motorcycle in races.