I'm thinking there is more to it slowing down than the convertor, boost likes tighter convertors with the boost ramping up faster has to equal to faster ET maybe your motor isn't putting out the same HP it did valve springs getting weak maybe some of the valves are leaking check into the motor make sure it performing correctly on all cylinders.
I’m not disagreeing with Al loose convertors makes cars faster but with boost your car should of pickup more et and mph at least mph something is wrong with the motor in my opinion.