My Corsair is also mounted "upside down." My case does have a filtered grill opening downwards, but I installed it upside down anyways. It just seemed more logical to me for several reasons.
The case is basically divided into halves by a big graphics card right down the middle.
In the top half, there is optical drive and SSD in the front, so minimal heat and plenty of space. The RAM with its heatsinks and the CPU with its cooler sits in the middle, and their heat is taken out of the case by a fan on the back of the case.
In the bottom half, there is an front panel fan drawing air in past the two 3.5" hard discs, and there is also the GPU fan blowing air DOWNwards. So both the front fan and the GPU fan are blowing air towards the PSU. There are no rear panel fan in this half of the case, although I guess you could install a PCI slot type fan which could draw air out of that half of the case.
Since I didn't put in a slot fan, it seemed more logical to me to mount the PSU upside down so the air coming from the front fan and GPU fan can go thru the PSU and out of the case.
I guess it might make the PSU overheat, although the airflow would be a mix of the cool air from the front fan with the warmer air from the GPU fan.
Maybe the next time I upgrad, I should flip the PSU, and install a PCI slot type fan?