You guys are crazy. :-)

The strongest pump I can find in local watercooling shops is the Laing D5 Vario. Those other monstrosities are from gardening-sections in DIY-stores, I assume?
A package containing a second MCP355 is on it's way and I'll see how far I can take flow-rates with that. If I don't quite reach a gazillion litres per second, or whatever 3 GPM converts to, I'll consider further options.

Thanks for the positive feedback everyone!