I agree. Plasticard and cardboard is much better for vehicles.
And definitely cheaper too. I can't see much use for
GS other than really fine details (not very orky) and gap filling.
Maybe you could also use some templates for
40k paper models as a basic shape for your vehicles and orkify them.
Here is a long list of different examples:
http://www.4shared.com/dir/2PQOMQdP/Eli_Patoroch.html#dir=2PQOMQdP You could just print them out, glue them on cardboard and build away. Or if you want them more durable use plasticard of around 1mm thickness. But you'll have to change the template a bit as you have to keep the 1mm thickness in mind for corrected measurements.