my friend told me this "Iwaki would have roughly 4ft of head pressure while the two D5's would have around 3.75ft of head pressure"
??? Iwakis have like 21 feet of head dude...this is of course with nothing in the loop at all so it'd basically be read as 21ft of head at 0GPM...you'd need to look at a PQ Chart to find out what their curve is after adding in blocks, heatercore and things like that.
A D5 is like .....I think 11feet...can't fully remember offhand, but I don't know what your friend is talking about in regards to 4ft...even after adding in the blocks and things like that it won't knock it down to 4ft.
I've attached a PQ Curver for the Iwaki family...the 55RT obviously is the one I'm talking about thats the powerful 21ft of head pump...it wouldn't be doing 4ft of head pressure until it was having to try and pump out approx 17.5Gallons per minute.
According to that chart though it's actuall around 27ft of head, not 21...but Iwaki was something I'd never buy because of the price so I couldn't remember offhand.
EDIT: Ok scratch that, I can't attach the file due to size so go here:
Scroll down a little bit and you'll see the chart from a guy named Nikhsub1...he's also at xtremesystems and he knows what he's talking about in regards to water cooling, nice fellow as well....notice however this was posted in 2002 so things are definitely different, different models and such, but I was just giving you an idea of a powerful Iwaki pump and what their PQ Charts look like