"Whiplash" is caused by any dramatic movement of the neck beyond it's normal range of motion. Most commonly, it's hyperextension (backwards), like you would get by being hit in the rear. Permanent/significant neck injuries from movement in other directions is a lot less common, mostly for anatomic reasons.
The reason high back seats are mandated is to help prevent this kind of injury.
I use a Cobra Clubman seat, as supplied by Factory Five. It provides good support on the track, and is also very comfortable on a long trip.
Clubman - Sports - Cobra Seats
