is it still effectively 2/3rds? Don't know enough to compare a discrete GPU/CPU setup to something like this where it's more integrated

There is no magic, if the data you compute as atomic chunk don't fit in cache then memory bandwidth R/W limit kicks in and architecture does not matter. On contrary - having multi gpu setup of same price and same memory size with even slower memories may give you effectively much higher bandwidth but at the cost of power consumption.