'Dump valves' dont suit any car, really. The proper term is air bypass valve and in keeping with most turbocharged cars built since 1980, the 944 Turbo already has a recirculating air bypass valve.
You can end up with a situation where the standard part isnt up to the job, but vent to atmosphere valves are all about showing off and very, very seldom required. Unless youre fitting a VTA valve with a duckcall, in which case it will be perfectly acceptable and quite funny. On ScoobyNet I always used to advise eager new members asking about 'dump valves' to leave the car as it was and simply rev it at traffic lights whilst shouting "Im a c*ck!" out of the open window. Same effect and your car will still run properly, which it wont with a VTA.