It is not a good idea to work this problem backwards, you end up with odd velocities. Start with the upstream pressure (114.7 psia) and apply the critical flow equation and the maximum downstream pressure that will give you sonic flow is 60.56 psia. Any pressure less than that number will give you exactly the same mass flow rate, velocity, and volume flow rate.
In your example, 27.8 psia upstream of an exhaust to atmosphere would give you choked flow, but since the upstream pressure is so much lower, the mass flow rate, velocity, and volume flow rate would be a fraction of their values at 114.7 psia upstream.
With regard to your salesman telling you that the velocity was less than Mach 1.0 there is a time that that number would be right--if the valve configuration forced a pressure drop between the valve seat ant the actual exhaust then you would never see pressure below critical until the exit plane of the exhaust pipe. This could happen if there is a lengthy tail pipe or a tortuous path through the valve. In this case, the pressure immediately downstream of the valve seat could be slightly higher than 60.56 psia, and the remainder of the pressure drop is taken incrementally through the ports and down the tail pipe. If that happened then you could be a a fraction of Mach 1.0, but I've never seen a valve with that much pressure drop after the seat.
