360 single rad or 240 + 120? That would make a difference.
Also, I think the purpose is to make things relatively simple for most people, I know this forum is full of mad men that can fit LOTS of stuff in a tiny space.
EK PE360 Dimensions: 400 x 130 x 38mm (L x W x H)
You can't just take the rad dimensions and add 10mm of clearance. There are a lot of things to consider, like manufacturing (ie: Velka 5 is made by bending sheet-metal while the P1 should have a CNC frame)
Also, unlike the Velka 5, the P1 won't...
360 rad in 14L is not possible, at least not in a user friendly manner.
Also, the case will be about 490mm (19,3 inches) in height. It will start to look like a tower and might need a larger 'base' to be stable. 360 rad is overkill IMHO.
It depends. If the P1 ends up supporting 280 radiators, the volume would probably be around 14L. Otherwise, if it supports 240 radiators only, volume should be about 12L. At this point only Wahaha360 knows.
For reference the NZXT H1 is a 13.5L case
Personally, I'd be totally okay with 240...