I believe that in the colder parts of Aus., they remove the QX prior to winter. I never have to because my queens keep laying right through winter.
Adding an extra super is an added burden to a colony. Something will have to give initially. They will work on the new super as well as work on the flow super, but I guess the production into the flow super will slow down considerably.
The thing you need to consider is: are you giving them a box full of empty frames with starter strips, fresh foundation, drawn comb or a combination?
The best advice I think anyone can give you is to find out what is working for your local apiarist. Be a sponge, soak up every bit of information he/she is prepared to share with you.
A little story: A while back a forum poster picked up a nuc from a local supplier. The supplier gave him/her some advice. Then that person asked the forum if that advice was right or not. Chances are that that advice was right. If that local supplier had nucs for sale, he/she must be doing something right.