I've been reading a number of articles, blogs and forum discussions on tuning backups by using the 'NUL' disk device. Since the nul device is a construct within DOS/Windows/Linux, the "bit bucket", data you send to it goes nowhere and is not written anywhere.
So if you are trying to tune a backup and see what the theoretical throughput is what exactly are you measuring? It seems to me this is measuring how much can be read from disk, pump into memory and pump out to nowhere.
How far through a system do we get before we get to nowhere? Right up to the point of going out to disc? Then correlate this to whether the disk can handle the throughput using a tool such as SQLIO or CrystalDiskMark?