assuming the heat from the PSU is rising up, wouldn't mounting it upside down cause the heat to stay in the vicinity of the PSU circuits?
Well, IME/IMHO it would look like a somewhat rough simplification: first of all, as air is a gas, then it will go through wherever it can, so through the back/side panels, which are at a convenient height from the case bottom, and not stay there.
What it's true, indeed, is that PSU will arrive wrapped into a warning sticker saying to mount it with the top grille facing up (without saying a word about why and when): in my experience (I own a 460 and a 750) it doesn't matter mounting the PSU either facing up or down (they are all alive'n'kickin after some years) but, you know, that's just my personal experience.
Then, as said by Vicotnik, you must specify how much wasted heat would stay there (at most the X-400 should waste about fourty W, however). Eventually, we can't assume that the air is stagnant, because a starving airflow is secured even by three slow running case fans.
Last but not least, as IMHO there should be about a little difference mounting that PSU straight or flipped, whether your primary concern is to lower the hard drives temperature, more probably that not you should mount *any* fanless or semi-fanless PSU facing downwards (to be honest, IMHO that requires also some testing, in order to determine whether an 80Plus Gold PSU orientation may significantly impact the hard drives temperatures, which usually mostly depend on the drive itself).
Summarizing, IMHO: regardless of PSU orientation, there's no worry about heating the PSU itself under any reasonable usage conditions (such as up to 30 hours continuatively at high to full load without any ventilation, as that's how SPCR tested the X-400), and there could be very little about heating the hard drives, which could be somehow addressed facing downwards *any* fanless or semi-fanless PSU.
Why not using the case fans for moving away the hot air around the PSU? (I clearly remember reading something about this in some fanless PSU review, but I didn't understand the idea back then either..).
Because in the specific case of an high quality 80Plus Gold rated unit, there's about no need, providing your primary concern is to keep as low as possible the hard drives temperatures. Obviously, it won't hurt, either: definitely, IMO you may mount it as you may rather (as you should get comparable results).