It is better to be above than below.
Mountune have had issues in the past when there return was below
So they moved them above and the issue was gone .

There are a few more factors to take into account but
In a few words it is better to above rather than below

Many years ago I had a frst and the return pipe was horizontal
Just before the pipe reached the block it kicked up a touch this did cause the
Oil to back up and the turbo to smoke its nuts off
( I was told this was common )
So I re positioned the pipe and the result was no smoke
Point being even tho the turbo was a little higher than the return it did not take much for the oil to back up the pipe , but that said
If I remember right the return was far smaller than most these days