I know you said you checked them. Just thought I'd mention that with my Z1 tb's I can not set the idle low enough either. Too much air gets past the throttle plates since they don't seal well enough against the bore.
You can do what I did to verify. Remove the intake pipe leading too each throttle body and seal the throttle body with some sort of cap(I used a coupler, two clamps and a PVC plug that fit into the coupler snugly). You should plug the two open pipes coming from the intercoolers aswell so the air fed into the engine is drawn through the maf. Since the only air getting into the engine at idle should be through the IACV you can now start the car and see if you can set idle properly. If you can then obviously the tb's are just letting too much air in.
