This may not help you, but what we do here when dealing with tray breaks/multiple columns is we break up our excel database. Say we have a 4up postcard with an 4000 person database. Instead of using the repeat feature in PSM and one set of variables in our database, we use a four-up image of our postcards and four different sets of variables. (Instead of 4000 records in @address@, @name@ etc, we have 1000 in @address1@, 1000 in @address2@, 1000 in @address3@, and 1000 in @address4@.)
Now this requires a bit more work at first, (step and repeating your document to the proper size, and splitting up your database and adding the appropriate fields) but if you find yourself losing a lot of time sorting after cutting, it could really help.
Not sure about slipsheeting, as we don't use it, but I hope I could have been of some help.