Bosh arent made for the extremly hot ignition systems GM uses. They just burn up after 15-20k miles.
GM does recomend the AC Delco Iridiums which are $6 each but will last you 75-100k miles.
And GA's are easy to change to plugs compared to the Grand prixs 3.8L. You usually have to loosen an engine mount to tilt the engine to get to the rear plugs :banghead
hope that helps.