''So do the quivertip ratings (1oz, 2.5 oz etc) relate to the casting weight or is it the "test curve" of the tip?''
I am not sure mate, but I use it as a rough guide to casting weight, although you can often cast more!
I would say set up your gear and fish light to start with, in comparison to the rating on the tip, then step things up slightly to say a bigger feeder or lead, this way you get a feel for what is what and you will understand what your tackle is capable of.
I have cast out feeders much heavier than the weighting on the tip, its just a case of knowing your own gear, and taking your time.