Any bypass valve will work vented to the atmosphere if tuned properly. When I had a P1 SC I had a vortech mondo bypass vented to the atmosphere. Way too freakin' loud for me.
A good tuner can use any meter to tune a P1SC, blow through or not. I used the stock meter on my car with a procharger. Procharger has their own bypass valves that can be vented to the atmosphere so you might try there.