However I don't know how much of a downside there is to having a PSU that has much more power than required, particularly at low loads (like using Word or Excel)? I understand that it can be less efficient when using lower power, but how much of a downside is that? A lot? Is inefficiency the only concern? I don't know, and am not an expert on PSU's. See my own discussions on this issue
As you may be aware, while the Corsair AX850 is cheaper than the X-1050, it seems quite loud when you compare it to the X-1050 (25db vs 16db @ 400W, 35db vs 16db @ 500W). You may never hit that level of power usage, of course, however even at lower levels there is still a disparity between the two (AX850 is still 5db louder at 300W), so it really depends how safe you want to feel.
Looking at how the X-1050 fits in to the current case I'm using (which is itself not a very special case) the bottom fan seems
to more or less line up with the holes in the bottom, however in this case it was a bit difficult to tell due some of the constraints this case imposes in fitting the PSU into the case. From observing pictures of the inside of the R4 and considering the length from the end of the PSU to where the fan starts, I would estimate the fans should mostly if not completely be over the holes in the bottom of the case.