Good points Ivan. I did actually put some little self adhesive feet on the down side, which raise the NAS about 5mm. It wasn't for the sensible reason that you suggest though, it was more the fact that I didn't want to scratch the unit up when laying it on it's side!
In terms of drive temps, you're absolutely right. Disk 2 is running at 35C and disk 1 at 36C so I assume it's for the reason you suggest, although off the top of my head I'm not sure which is the upper disk until I physically check. I think that the 80x80mm fan that is running at all times is literally sucking the warm air right out pretty good.
Overall, i'm really happy with the temps, though.