18 front (1 up) corrects the speedometer but predictably, the odometer will then be off.
I rode with an 18 for years, has no problem two up with luggage in the moun to ain switchbacks of TN and NC.
A little slower off the line reflected by 1/8 mile times, but 1/4 mile times the same as you can pull 4th gear through the traps as opposed to running out of gear or short shifting.
Personally, I enjoyed both the slightly broader usable speed range of the lower gears and felt I needed to shift less frequently. I also liked the cruising rpm a little more relaxed. Fuel usage improved by 1-2 mpg overall.
Your mileage in all these aspects will vary, of course, depending on how aggressive you wish to be with the load handle.