The fumes are tolerable. If they're really that bad then your car isn't running right. You do actually lose a veeeeery small amount of torque but you pick up a veeeeeery small amount of HP too. The gains/losses are irrelevant but they will show up on a dyno, but they're not good enough reasons for or against running tailpipes. Your car will also be quite a bit louder all though the RPM range.