The observation about the fan blowing air onto the transformer in the original manufacturer configuration is correct, a poorly designed system. The thermal sensor is located on the upper heatsink and receives little air from around the transformer, which gets already warm. I did some tests by raising the temperature of the whole PSU to 80 deg C then timed how long the fan took to cool the unit back to the fan cut-off temperature with no load.
1. with fan blowing inwards 16.5 minutes
2. reverse fan, now blowing outwards 7.1 minutes
3. reversed fan with the top ventilation slots covered, both side slots open normally 5.8 minutes
Not only the heatsink was cooled more efficiently with the fan exhausting hot air out the back, but the front end of the cabinet came down to ambient temperature as well, which it never did with air being forced inwards.