This thread is FULL of BAD INFO for AIO's.
PLEASE and i say PLEASE go look at Gamer Nexus's AIO orientation Video linked below, before you overheat your CPU, or permanently damage your expensive AIO's pump.
Do not mount it that way. Steve explains it at 18:00 in the video above.
It can lead to your pump running dry or partially dry leading to possible damage in the cpu pump.
It was explained in detail with gamers nexus.
The only way you can mount it like that is if your using a true custom liquid cooler which has been completely bleed,
which no AIO's are.
Always keep the cpu block either at the lowest or middle point in the height.
Always make sure the Barbs are not at the top in a vertical standing config.
You want them at the lowest point so air does not get pulled with coolant.
And NEVER lay the radiator on the floor horizontally in a AIO, as you WILL pull nothing but air and burn out that cpu pump, as well as have really poor performance.
Fan Orientation:
Radiator Fan Orientation and TFC Shroud Testing & Review
www.techpowerup.com
tldr;
It plays very little performance impact.
Its mostly on maintaince... its easier to vacuum the the rad fins when you have nothing obstructing them then if you have a fan covering them, which is why i am a firm advocate of PULL and not PUSH, even tho push gives you a slightly better performance curve.
But remember that performance curve will get SHOT if its full of dust which you can not remove, because you have fans blocking the way.