I haven't found an answer yet, but this might be the discussion where you read HA was more important than DR (for training OD anyway).
(301522.9)Two other notes - 1) the elastic effect wozzvt found was between OD and HA. Driving didn't have a significant effect once handling was controlled for. So I agree with your last statement.
There is also a table at this link, although I'm not sure what the source is for these numbers:
(307729.7)
Trained skill -> (is towed by) Matched skill -> drag coefficient
JS -> DR -> 0.011
OD -> HN -> 0.007
HN -> OD -> 0.050
DR -> HN -> 0.005
PS -> HN -> 0.030
IS -> ID -> 0.001
ID -> IS -> 0.020
RB -> IS and ID -> 0.20 and 0.010
JR -> JS ->?
SB -> ID and RB ->?