I have cut a lot of fiberglass on waterjets. It will delaminate a small circle around the hole pierce, this can be avoided by drilling thru holes where they can start the cut. A backing board will not prevent it, it is a consequence of the waterjet pressure forcing between the laminate layers before the pierce is complete. The edge will be very nice and clean. You can also cut the panel in a conventional CNC machine, however fiberglass is pretty abrasive and so many CNC shops don't want to deal with it (dust gets into the machinery and wears it out). I will cut it on my open mill so I can vacuum up the dust and chips as it is cutting (but I still don't like it), I will not put in the VMC with coolant etc. - no way to collect the dust.