I agree, my stage 1 sierra highflow did very well for what it was but the response wasn't quick, at say 4000 revs it would take twice the time to spool as the GTX, possibly because the larger housing allows more energy to spin up the turbine, and less restriction? I wasn't expecting a faster response anyway...
It would be good to have some direct comparisons, especially if as many factors as possible are controlled. Same car, same dyno would be good but realistically it isn't going to happen. The more information the better.