Shutdown time is directly related to drivers and applications that are running when you shutdown. The only way to speed it up, is to find the driver/application that is taking longer than it should to shutdown.
Although Bootvis doens't help an SP2 machine, that is if you are running SP2, it might give you some insite into the issue. You need to get a trace of what is happening at a time when it's pretty hard to monitor a system.
I suspect that the problem is a driver, but it's possible it's an application that just doesn't want to die.
Something else you can do is this:
Change the WaitToKillAppTimeout value in HKey_Current_User\Control Panel\Desktop to something smaller. should be at 20 (milliseconds).
Also read this and see if it applies.