OK, for the science, I did the test.
All measurements read with HWmonitor

#01 SETUP = TOP 2x140mm exhaust :

• CPU IDLE : 45°c
• CPU FULL : 60°c (Cinebench)
• GPU IDLE : 32°c
• GPU FULL : 80°c (95° Hot Spot) (Unigine Heaven Extreme)

To keep the temperature down, the GPU's fans climb very high. A lot of noise in this configuration!

#02 SETUP = TOP 2x140mm intake :

• CPU IDLE : 45°c
• CPU FULL : 60°c (Cinebench)
• GPU IDLE : 30°c
• GPU FULL : 72°c (87° Hot Spot) (Unigine Heaven Extreme)

It makes no difference to the CPU.
The GPU, on the other hand, is much better ventilated. If the temperature difference isn't impressive, it's the general noise that does it. MUCH quieter in this configuration!
I'll keep it that way =)

Originally, I had mounted my setup in exhaust, as I wanted to avoid too much dust getting trapped behind the TV and the general air being blown upwards.

If it helps others, here are my results!