The calc is inaccurate, see ohms law…
As one example, with 64.8v and 98mOhm, max possible motor current is 661.2a, not “162.6a”
I suggest starting a new thread since this has gotten far off topic.
But to summarize the answer to the OP’s question, 10-12lbs additional thrust @ 15mph is plausible by dropping the motor resistance from 70mOhm to 20mOhm with a larger, same kv motor and changing nothing else. I assumed 2 motors, 12s, geared for about 40mph, with 30a bat and 80a motor limits per motor. At 15mph, they’d be hitting the battery current limit and not the motor current limit, and essentially the lower resistance motor has more motor current at that speed than the higher resistance one on account of lower I^2R losses (both have less than 80a motor current available at that speed on account of the 30a battery limit, but the lower resistance motor’s motor current is higher).