I cut out grains 8 wks ago. I haven't really been "training", but I do ride regularly. I wasn't really planning to do any XC mtb racing this season, but I did end up doing one last week. I was STOKED that my time was even better than last year when I was training.
I think it's a lot to do w/ cutting the grains out - I feel lighter, like I have a better power to weight ration & I don't have any energy dips any more.
I don't know how mtb fits in with "primal" but I just thought I would share my mtb experience with you. (granted, in Texas, it's really "off road riding", since there are no mountains, except in the far west & Big Bend).