True. If you get the chance though, I'd try it out nonetheless. Convection (heat rising) in PC cases is not strong at all and even the weakest fan should be able to overpower it without difficulty (having said that, it's still going to lose some airflow going against convection, albeit not much).
Try it both ways, see what gets you better temps, if there's a measurable difference at all. If you can get away with positive air pressure I would prioritize that. Provided you use intake fan filters your rig will be much cleaner. If you can have positive pressure with the three top fans blowing out I would do that though (EDIT: meaning you have other intake fans not pictured above).
Otherwise looks good, reservoir above and before pump is the most important thing.