A couple thoughts:
1) Logging at a faster rate takes away available processor time from other tasks. I am not sure how much "extra time" is available for use... be aware that increasing logging rates may cause performance problems.
1.5) There are ways to determine if performance is suffering (I think the keyword is "Performance Monitoring") but they may be difficult and I personally can't give any advice.
2) If you're committed to changing the logging rates to see what happens, try modifying is the scheduler task table in ArduPlane.cpp.