It definitely saves time from an engineering perspective if you take an original part and only change the outside profiles. Though there is still a lot of work involved it's not usually as much as starting from scratch.
I think the description of taking a transformation design and rebuilding another vehicle/bot around it is quite apt in this case, even though it's a completely different assembly of parts isn't the torso of ectotron a very similar shape/structure to that of onslaught?
I still haven't opened mine, might happen one night this week.