This is interesting. I wonder if the problem is that the "regular" 528 is for non-1.6 toilets? It seems like if the toilet flushes 1.6 gallons, then it needs to refill 1.6 gallons, no matter what brand. Possibly the Toto toilet bowls hold more water ( bigger spot) and hence take longer to refill. Makes me wonder if they are really using MORE than 1.6 gallons per flush!