yeah... The collision box is based on the absolute center via measuring the furtherst vertex in each direction of your model.
This is how a bounding box is calculated. So - Either you toss in an invisi vertex here or there to force the move (kind of hokey).
you could make your model not even seen by sparky but have a cube (invisible) set for sparky and position this invisi cube (alpha zero might do the trick - I THINK it needs to be technically visible - I dunno) But position this cube at the posityion of your model - and offset it's position so that cube surrounds your model the way you want each frame.
-Jason