Throw bag and rescue rope are 2 different things. If you need a wrap kit, get a real rescue rope. That said i have certainly set up a few z drags wih throw bags. Problem is the rope isn't meant for large loads, throw ropes are typically dynamic, ie. you have to pull the stretch out of the rope before you affect the load & then the throw rope is typically all bunched up inside the sheath... Throw bags typically take a lot of abuse which is not good for a rescue rope,... So what are your priorities here? If you want a strong throw rope spectra is strong. If you want something for a wrapped raft you should put together a wrap kit also with static rope.
If you are using a throw bag for a z-drag, be aware that the rope is not as strong as you may want it to be, and consider that the core of the rope may well be compromised as well; stepped on, sand ground into it, stored wet, general abuse, etc..
Another thought on throw bags is don't get the thin guage ones, too hard on your hands. Think mine are all 3/8"