mine runs fine on setting 4 on D5
But better head and flow rate do not hurt so get the Iwaki if you want.
I think the D5 has 12' head max and 1200 l/h flow rate. However, don't be fooled its head max that's most important.
The only thing about the higher in Iwaki that you need to be concerned with is the added heat. From what I recall reading in other threads its some where around 50Watts. However, if you use the larger rads (that need 3 fans each at least 75 CFM each) you should be fine. I wish I could buy one here in the states.