The dyno shows gains starting by 2500 rpm.
I don't have a Mastermind, but I do have a quad resonated catless x-pipe and I felt no loss of low end power. In fact, it feels to me like the power is stronger everywhere. I do not have any dyno results to back up my feel, but my feel is the opposite of your feel.
If I had to criticize the Mastermind, which looks really nice, it would be that the center of the X reduces the flow because the X looks like it is designed to force one flow into another and to squeeze both into one pipe (that flows less than the two separate pipes) and then cross over and flow out the opposite pipe. My understanding of the need for an X pipe is that it is just a balance pipe that gives the exhaust gas flows the opportunity to kiss and touch and balance any pressure differentials, and that it is not to actually force one flow into or through another flow or to reduce the size of the exhaust pipe. I prefer the curved X pipe, where the pipes just touch. I would like to see a dyno of a few different x-pipes on the same car, but doubt anyone will bother doing that anytime soon, not that it would be very difficult.