The OP question was for 1 card in 1st or 2nd slot. Let's not change the subject.
I Didn't !..... if you really analyse the question, running on air ... moving even a single Video card away from the CPU cooler increases the distance between the two ......increased distance means a decrease in radiant heat transmission (Infrared Waves), increased air volume between our little heat generators,.... allowing our fans to do there thing more efficiently...... it might sound silly but small, incremental, numerous steps taken , combined can have a large impact on a system.
As far as video card performance goes........I doubt if there would be 1-100th of a single frame rate in it if there was such a thing,.... but 2 degrees.....Maybe!!
Have fun.......Aussie Allan