Normal
Your setup is a negative pressure setup (no intake fans, just the cpu cooler as sort of one) so air should flow past the PSU and CPU both. You're probably right that an L12S would hinder airflow across, but mainly because its fan being bigger and closer to the vents would serve more directly as an intake (or exhaust, if you flip your other fans). Then again the 120mm fan might be powerful enough to push air out/pull air in both sides.
