Accel boost is a modification that can't be added before delivery, but FSD can be added before delivery? That's strange.I asked my SA this same question and she said:
We can not add it before delivery. It is considered a modification which is why it must happen afterwards.
Oh well. Although I do want the acceleration boost upgrade, now that it can't be added before delivery I thought I'd drive the car for a couple of weeks then add the upgrade so I would hopefully be impressed with the difference.