Many cases, in fact almost all reviews done on cases where airflow is looked at, are only done using the stock, factory installed fans. With gaming pc cases, that's usually just 2x 120mm. Of course some cases come with 3x intakes, and forget to use an exhaust. So take airflow with a grain of salt, especially if you plan on changing the fans/amounts. The p400s doesn't really have bad airflow at all, once fan mounts are populated.
I own a FD define R5 Window. It's got a door in front of the intakes. Ppl say this is a detriment to airflow and cooling will be bad. As is, that's where my kraken x61 sits, the stock 2x fans as top exhaust. No rear exhaust. Cpu temps changed a whopping 0°C at idle and rise 1°C with the door closed. Fans sit at 450-650 rpm.
If you like the p400s or the 500i, then get it. Cases are personal, you gotta look at it and live with it we don't. But fans, curves, speeds, can be changed, which will change airflow. You don't need a gale inside the case, air just has to move in/out smoothly, not spin around. So the amount of air moved is of far less importance than how it moves.