First on the subject of the high HP racing engines. We don't run our pumps at the same ratio as a street engine . Mine is reduced to to 50% of what stock is so it runs at about 4,250 RPM . We also use speicial pumps that prevent cavataion both water and oil pumps. So thats really not part of the equation with the use of restrictors.

I have used for pumps so far the rd20 /rd30 and the alphacool ap1510.

your correct as is normally the case that the RD30 in paralell will get me to 10.6 gpm but 2 of these dump a lot of heat into the system and are noisy as hell. 2 rd20 are good for 8gpm. but still to loud for me than I have to convert 12v to 24v on the RD20/30 which is ok.
Right now the Alphcool AP1510 is what I am happiest with . because of the v.control unit . It pushes around 1500Lph which is right at 8gpm . which will in fact do the job . But I really would like about 10gpm and cut v back so as to have a little overhead. the 1510 has excellant head.
I did have to resize the 160 for 3/4 inlets and outlets to maintain good flow however. Which is OK . I am ripping off that design because we can make them cheaper than I can buy them . When I can find them . Than we also make them showy looking .
I won't be reselling these as I wouldn't feel right about riping off someone elses design. They will just be used in our gamers.

Besides with the added work of making them show piecies I couldn't compete price wise.

Now off topic . I laughed pretty hard on the story you told about the nurse ar [H] . I have a good one also.
When I got injured in 82. broken left shoulder completely dislocation of the left knee (only thing holding my leg on was some skin )ruptured L4 and L5 in lower back and one hell of a headache. I had this Candy stripper bring my food . Well she would come in lay down my tray and leave . Here I am with only one usable arm/hand . Well I didn't eat much first 3 days anyway.

The 4th day she brought it in and was heading out the door and my tray hit the door befor she got out. Dam the doctors were coming in to see me at that same time. He yells at what the hell is going on . I yell back in not so good wording . LOOK AT MY---- bread and My GOD --- milk . Bread isn't buttered and milk isn't opened. They went out in the hall there was some yelling . Doc came back in and said please except our apologies she no longer works here.