drive at 70mph then management light comes on?

Doing 70mph on motorway suddenly engine management light comes on then speed starts to drop to 60mph even though im full on the gas. My foot was down on accelerator and 60mph was the max i could get. Also i know that my exhaust middle box may have a hole-would it be possible that the exhaust is causing engine management light come on+speed to decrease and not drive more than 60mph? (vauxhall zafira club 1.6-2003)