It should be fine either way. IIRC, the side fan is supposed to be an intake and you've got the PSU and a few vents at the back for exhaust, right? The CPU cooler and GPU won't change the equation, so the 120mm fan only has to be stronger than the PSU fan in order to make sure that you've got positive pressure so neither PSU nor CPU recycle hot air.
With SFX, this is a no-brainer. They often use 10mm thick 80mm from what I know, a decent 120x15 should generate much more airflow. With an SFX-L I'm not so sure anymore, but those fans don't run even close to full tilt, no matter what, so even then it should be possible to balance that out quite well.
The 4L, 5L, 6L and 7L marks are always a psychological barrier for some of us. You don't care whether your case grows from 6.7 to 6.9 litres, but you care very much when it goes from 6.9 to 7.1. It's like you've lost a fight with space itself.
That's why my case is currently 4.957L and [USER=99]@Hahutzy[/USER]'s is something like 3.994 and maybe it will be why yours stays 5.9 
