I do not use any wad pressure for ay load using a plastic integral wad/shot cup - it isn't necessary. Wad pressure is a holdover from the old days of felt wads and since folks still load with them and shot overcards, etc., it can be useful.
Dave - you might want to call ClayBuster - they'll send you a sample of 25 to try for no charge of any of their wads. This pink one is giving me some great crimps and performance. I am wondering if the chrono'd low speed was due to the gun bleeding off too much of what pressure/gas there was in order to make it work......but I am also going to bump the load to 17.5 - which should get me where I need to go. I will also chrono my current loads in my O/U to see if the speed stays the same