Cathar,
For free flow conditions, I believe the E-1048 is designed to produce more flow but with less head than the CSP750.
E-1048 => 158 GPH free flow, 2.2 PSI max head
CSP750 => 120 GPH free flow, 2.7 PSI max head
With little flow resistance the Eheim will out deliver the C-Systems pump in GPH. This was demonstrated in the Pump Flow vs. Tubing Length graph. The E-1048 had a higher flow rate than either the single CSP750 or duallys in series.
However when the system backpressure increases, the CSP750 eventually out delivers the Eheim 1048 as shown in the System Flow Tests (BIE, Maze3, etc.). Even though these are pretty free flowing components, they still generated enough total system back pressure to allow the single CSP750 to almost catch up with the Eheim and with two of them in series or parallel exceeded the Eheims total flow.
Yes, aluminum is always a concern but corrosion can be easily managed with additives and errosion... well time will tell. Aluminum automotive water pumps certainly seem to be able to withstand a lot of wear and tear and they operate at much higher temperatures and flow rates than PC pumps, frequently with a lot of particulate sludge in the coolant banging away on the aluminum walls...
