imo, falling of the track and respawing always slows down the gameplay and frustrates, specialy in games with weapons where an unlucky hit could sent you flying off the track and cost even more time (everyone who played Mario Kart will know what i mean...)
and about "invislbe walls":
you could always excuse them by having an "energy shield" around the track that keeps cars from flying off (if that fits within grips universe).
ad a little animation when hitting an invis. wall and you are fine