that bottom vent doesn't do much for people with carpet.
assuming i get a P280 (and i probably will), i'll have to run the PSU upside down.
as for top vents, if there isn't a fan there it serves as an intake for the fan in the rear panel. feel it for yourself on a P18x. that may be ok if the computer is out by itself, but if it's under a desk with a rear panel, it's just recycling warmed air.
Even without a bottom vent, the PSU is elevated off the case floor on the P280, as also on the P183, so turning it upside down is not mandatory. Even on carpet, you could put bigger feet on the case to get some airflow underneath.
Whether or not a top vent without a fan acts as an air intake, that is only the case (no pun intended) if there is a rear exhaust fan of sufficient CFM. It is possible to run a fan on the CPU only, blowing straight up, with air rising through a fan-less top-vent, and no rear exhaust fan. Obviously, this would be for a relatively low power system, but not all that extreme as one might think.
There is a basic principle of physics at play--hot air rises--and one can exploit that fact instead of fighting against it by trying to force air out the rear at a 90 degree angle.