You'd want Tesla to verify that. I think it has been even cheaper than that to add Ludicrous after the fact to a Model S P90D, but I don't know if Tesla has gone on record saying that Ludicrous can be added to a Model X P90D after the fact. Logic says yes, but this is Tesla.
It's not a dual charger, it's one 72A rather than one 48A charger. This is different from the Model S that comes with one 40A charger and a second 40A charger can be added. That's what is meant by dual chargers.
I believe Ludicrous mode requires the 1500A electronic activated pyro fuse, that's what enables the high acceleration beyond the 30mph of insane mode. So it should be retrofit table, but whether it's worth it for Tesla or not is an open question.
One wonders if, in the name of supply chain simplicity, every Performance model is Ludicrous-capable and the upgrade becomes the flipping of a SW switch. There is some precedence for this with AP today and Supercharger access in the past.