I have a Brison 5.8 that is several years old in my Ziroli Corsair. What you described about your throttle operation is exactly opposite from my experience. I checked my Futaba 8U transmitter and I have -40 expo on the throttle channel. Without the expo, the engine tries to get most of the RPM in the first half of throttle stick travel as you mentioned about your other engines. Otherwise, does your engine run good? My 5.8 has been super good. Here is a photo for comparison purposes.