You may be alright on the stock pillion if it's that short of a ride. I would say try it first, stay close to home but see how she feels after about an hour on it, then decide from there. I hate spending money if I don't need to, the reason I upgraded to the corbin was because I sometimes take long trips to FL or north GA. If my rides weren't more than 2 hours long, I'd never bother with upgrading.
But if you're going 2up, you need the corbin touring seat, not the gunfighter. Sorry.