We have talked about this with engeneering way back. In the "old" days you could not use a writable with a machine assignment because the first attachment had to be the writable dur to it being read/write. I know that they were trying to fix it as to be honest it doesn't care at what point the writable is being attached as long as it is marked as read/write.
Haven't tested it myself with the newer version to be honest because user group assignment worked better for us.
When looking at the actual attachment, it does not matter if you assign to OU, group or machine. The only thing we have seen to slow down attachment is when you are using large groups with thousands of users to assign a appstack or use specific machine prefixes to attach an appstack and use multiple groups for that.
When you assign to a machine pre logon does it really matter how long it takes to attach the appstack? You just need to make sure that it is there the moment the user wants to log in. Worst case scenario you need to have more machine active to accept connections.
