Well...leave apart if ImportObj works or not...
My workflow will be modeling the shape in sub-d,then import in MOI as obj just like reference for modeling tires, rims,wipes,handles,logos and so...
In this stage MOI is perfect :) so I'll export these parts into my sub-d app to complete the car and render
You talk about concept,but if you want to make this concept believable you must add details on it,or it will remain a "3d sketch"

M