Nothing wrong with a 120mm PSU in a P150.
The problem with a 120mm PSU in a compact case, like the P150/Solo, is that the PSU fan and the rear case fan, are in such close proximity, they are fighting each other for the same air, therefore conflict between the fans, leaving both fans not working to their full efficiency. Also potential conflict with the CPU coolers air flow. You could build a shielding duct over the rear case fan, towards the CPU cooler.
Replace just a few words and you get a common problem with another case:
The problem with a 120mm top exhaust in a big case, like the P180, is that the top exhaust and the rear case fan, are in such close proximity, they are fighting each other for the same air, therefore conflict between the fans, leaving both fans not working to their full efficiency.
The solution chosen by most P180 owners- remove one of the redundant fans and cover the hole. Since the point
of a PSU with a big bottom mount fan is that (in a case with a regular layout like the P150) it'll pull the hot air directly from the heatsink area and exhaust it from the case. It's what they're designed to do. Just skip the rear fan, cover the hole, and let the PSU fan do the work. Admittedly, that's going to put some extra heat into the PSU, but he is
using a nice cool C2D. He could even duct that hot vid card to the rear exhaust hole instead of covering it up.