As emitfudd said, you're choking the AIO's fans.
Fans and placement. That's what causes the gap between AIOs and air coolers to either narrow or widen.
Air coolers are always near the center of the chassis so the main source of air resistance for its fans is their own heatsink. The fans almost always have breathing room, unless there's too much plastic crap on it or something.
AIOs, placement is important alongside the fans. An AIO's radiator is a greater source of air resistance than an air cooler heatsink, and combined with the solid/semi-solid panel, you've now stacked up on the air resistance. You have to make up for that by running the fans harder, but that comes at the cost of noise - or don't, and cooling suffers; can't cool the cpu on the liquid flow alone.
Open mesh panels are the optimal location to place AIO radiators. Unfortunately, the P400S doesn't support AIOs at the top.
An air cooler is going to be more efficient here.