Just to further confuse you...
A higher threshold will always work wonders in cross. The bigger the aerobic engine and lactate tolerance/clearance, the faster you can go during the race. (think 2x20' workouts, etc)
Higher VO2 Max power will help you with 'surges', like a hot lap to prevent someone from trying to pass, or catching a group after a bobble. (think 3x3' intervals)
High muscular power (sprints and mashing a big gear) will help you over the hills without incurring as much 'oxygen debt'. (sprint workouts)
All the above is really just a fancy + complicated way of saying you should probably work on exactly what you're bad at. Find a hill that takes 30 seconds to climb, and do something like 3 sets of five ascents at max effort with ample rest between. Maybe work up to four sets of six.
Also, do you weigh too much? (what bike racer doesn't?), but the more you weigh, the harder the hills are.