The manufaturer's name may be a pun on Pussyfister, which refers to some one who shoves their entire fist in to a vagina.
This manufacturer in theory would be owned by Vulcar, as Porsche belongs to the Volkswagen group.
In GTA San Andreas, pedestrians in areas like Paradiso and San Fierro can be heard saying what sounds like "Have you driven a Pfister?". This could mean that car brands were planned to be introduced before GTA IV.
In certain Stores of GTA V can be seen Pfister Design posters and articles, meaning to the Porsche Design of real life.