Safety Question on Cosco Juvenile High Back Booster

I recently was given a hand me down five point harness booster seat that was manufactured June 23, 2008 and the model # is 22277-vrt which expires in December 2014. Now I know that at one time or maybe even still that IIHS did not recommend it because of poor fit as a booster for kids. However, that is what my question is related to is that just when you use it as belt positioner booster, or is that also true if you are using it as a five-point harness as well? This seat would be used as an extra seat that nanny would use on part-time basis. Any help and or advice is greatly appreciated and welcomed.

Thanks,
Karla