or is this almost as good as buying cured rock?
Way better,
IMO. In a perfect situation, the rock would go from the ocean directly to your tank and require no "curing" whatsoever. It is only when the transport time is so long...that there is such significant die off on the rock.. that you need to cure it to deal with all that death.
The final challenge is to have a tank in place that can support life (fish,
LR or whatever). You could take
LR directly from the ocean, but if you place it in an uncycled tank, you're going to produce a deadly cycle regardless. An uncycled tank is deadly to the
LR, just as it would be for a fish.
LR will always eventually function as a good biological filter, no matter what it went through along the way. But there isn't much point in paying big bucks to have
LR rushed to you if you only want it to function as a biological filter.