hmm....
Well... <long winded and purely hypothetical argument follows>
Presumably the longer shocks on the longer springs (for the lifted motors) don't shrink down as short.. Which I would have thought is ok on a lifted motor, since you have more metal in the spring.
So whilst it would solve your problem, when you do compress the spring to it's fullest extent, are you not perhaps in danger of stumping the shock into itself (i.e. squashing it)
Of course, that's just an initial "hmmm" moment, ask Tim, he'll know :)