spends about 75% of it's time under 55mph with 1-2 times a month at 75-80 for a few hours.
without getting into some serious tire swapping, the effect of the gear change on the vehicles pull away will come down to a gut decision.
However you can simulate the engine noise / car feel with the gear options. Use one of the online gear ratio / rpm calculators to determine the engine rpm at say 55 mph and at 75 to 80 mph with your existing tire size. Then perhaps using a lower gear in the transmission hit that engine rpm in highway conditions for each gear set. You won't be doing the actual MPH that you listed, but the engine will be spinning like it was, so you can get a feel for it.
