Im in the same boat, car is running on only 5 cylinders, car is now parked in the carport and not getting driven till this is fixed! Im not sure if it would cause damage to the engine, but it is sure not doing it any good!!
Best price i have got for Splitfires is $600 dollars.
My problem is that i have had work done to the coil pack wiring loom, and this could be the problem.. Have to get another one and see if it changes anything, then change the plugs, then if that doesnt fix it im up for new coils.. the joys of cars hey!!