I have the backrest pad on mine, and never got a complaint. However, anyone who has been on the back has been wearing a motorcycle jacket with armor, typically including back protection -- i.e., it's probably softer than the trunk or the backrest anyway. The backrest pad does position her upper torso an inch or so forward, as noted above.
The best comfort accessory for any pillion taller than about 5'3" is a set of peg extenders, made even better if they're combined with a set of floorboards like Knifemaker (member here) has made and sold the last couple or three years.