I have the si II model (it's not high efficiency only 80% efficient, rated D).
The Cdi model (this is high efficiency, about ~ 90.5% efficient, rated A ) gives more hot water output for the same power.
28 si II = 28kw = 10.3 L/min
28cdi = 28kw = 11.4 L/min
32cdi = 32kw = 13.1 L/min...