Expansion chambers are critical to the way the motor works; it is very likely that the different pipes are the cause of the flat spot. When designing chambers, the first question asked is, "What type of riding will the bike be used for?" The designer will decide where in the rev range the peak power will be delivered, as well as how wide the powerband will be. For street use, the pipe will allow a spread of power to increase ridability; for racing, the pipe will be designed for maximum top-end with a relatively narrow powerband, requiring more effort from the rider to keep it in the power. Changing a dimension robs power from one part of the rev range and adds it to another; the effect of the tapered head pipe is to lengthen the diffuser; this narrows the powerband and may increase peak power.
Take both sets of pipes to the track and compare performance vs. your style of riding. If the track allows you to keep the rpm over 7000, you will have no problem with the Swarbrick pipes. If not, try the other ones. Perhaps the Swarbricks with a straight head pipe will be the answer for you. Experimentation will be the only way to find out for sure.