Remember it's not about the GPM, but the pressure that the pump produces and works against. The CM90 has a much higher pressure than the CM30. Now, if you want something in between the CM30 and CM90... get the CO90... Supposed to move about 18GPM at 0.25 bar.
Marine pumps don't measure the 'head' like fountain pumps, but they measure the pressure at which they can pump a certain amount of fluid (same thing, different measurement units i.e. - meters/feet vs bar/psi). The pressure you have in your system will determine the GPM. i.e. - if you have a restriction that creates 0.2 bar of pressure... then your rate on the CM30 will be about 5.2GPM. Check the chart
here. The CM90 is not on this chart, but you can figure it is above the CO90 as it's flow is higher at 0.25 bar.
Basically, if you keep the system fairly level, meaning the reservior/ice tank is the the same height or higher than the pump, then gravity works for you and the only real resistance you will have are the inlet/outlet sizes, intercooler and air charge cooler passages and a little bit of pressure to move the fluid to the top of the system (obviously less than 3 feet).
I realize that the whole system is in motion and there are variances, but you get my point.
Hope that helps... Cheers!