After some further testing, I realized that my Kontronik 600 motor was faulty.

Yesterday, when I was reducing the prop size, inch by inch, the current measurement didn't seem to decrease as I was expecting. Also, after I re-programmed the CC Phoenix HV45 for no current shutdown and no voltage shutdown, I was surprised when the motor still shutdown at 3/4 throttle. Finally, after replacing the HV45 ESC with a Jeti 77-amp Opto ESC and the motor still shut down around 60amps (Jeti has no shutdown mode), I realized that the high current draw with an 11x7 prop and the shutdowns were symptoms of a bad motor.

I replaced the Kontronik 600 geared motor with an AXI 4120/16 direct drive outrunner motor. Since I was stuck using a 6s Lithium pack, I decided that the 4120/16 will spin the smaller APC 11x7 prop nice and fast for the Ultimate. I measured 53amps at 1100 watts using my 6-cell BalanceProHD Kokam 3200mAh pack which can deliver 64amps continuous so nothing is getting stressed. My new power level is 1100w/5.5lbs = 200w/lb.