Back well over a year ago, there was a thread at Ars discussing the merits of not using a VHAR WU for a benchmark (it was a 6.7xx at the time for the 2.x & 3.0 clients, IIRC). The discussion revolved around the fact that the best benchmark WU would be one where all or nearly all of the SETI scientific analyses would be taking place (VLARs & VHARs don't have this), in addition to the most prolific WU.
A number of people evaluated past results in the Setiqueues and pretty much concluded that the most distributed AR WU was the "0.417". I know that for myself, having had a queue record of about 1100 results at the time, I found that ~13% of the WUs were 0.417. This was out of an ENORMOUS possible number of WU ARs (considering ARs have been distributed from a range of 0.000 - ~18.000, from what people have reported).
Thus the bench was changed to a 0.417.
Anyone interested in this fascinating thread may check it out
here.
[EDIT: BTW, I know that not every AR is apparently distributed as a WU, so the actual number of ARs that have been sent out is not the theoretical.]