Where does it say ISO 6801 in the article? I can't find any references to it. I know I had an error a long time ago, but that was changed to 8601.

Re: the setting of DATEFIRST. Note this section from my article:

First you should note that whenever the day of the week is checked for a specific date, we always make sure that the ISO standard of using Monday as the starting day of the week is used, regardless of the current setting of @@datefirst. This is accomplished by adding @@datefirst - 1 to the value returned by DATEPART(dw, ) and then dividing this number by 7. The remainder of the division is the day of the week for the specified date using Monday as the starting day of the week. Note though that Sunday becomes 0 with this expression, so if you need it to be 7 (for some calculation or whatever) you must exchange it for that.

Finally, regarding performance, the fastest way I have found to get ISO 8601 week numbers is to use the built-in routines in Visual Basic.NET. Creating a user-defined function in Visual Basic.NET is very easy, so if you're running SQL Server 2005 that is an option.