I don't think you're correct. Let's consider a battery pack with 2 18650 Li-Ion batteries. Suppose each cell is capable of receiving 3A cont. max charging current. It doesn't matter what charging voltage you use, because it will have to be changed with a switch mode power supply to equal a voltage which will drive 3A across the internal resistance of the batteries (Charging method for Li-ion batteries is constant current)

Since the batteries in scenario 1 are in series, we can only drive 3A into the pack, whereas in scenario 2 the total charging current can be set to 6A, which will charge the batteries twice as fast. I don't understand where you get the notion that battery charging is measured in wattage -- Wattage is simply a limitation of the charging setup if you don't have enough volts to charge at a certain amperage.