you get much less stretch if the plants mature when you flip 12-12, so you end up with bigger buds as the nodes stay much closer together so the buds join up with each other and form bigger single colas instead of a cola at the end of the branch then smaller buds at the nodes below, keeping the stretch down helps produce the bigger cola's, so i keep my plants in veg now until they have shown sex or at least till most of them have shown sex as sativas tend to take longer to show sex.
also how are you running the nutrients now you flipped 12-12, are you using the mix you mentioned or will you be using the veg nutes until you see flowers forming or at least till the stretch has finished then change to the flowering stage feed, i always changed to flower feed when i flipped to 12-12, but since read that plants need more N during the stretch so using a flower feed to early restricts the plant, which i guess does mean less stretch but might not be the best for the plants in the long term,
so ill either use the method you suggested and use a mix of flower and veg or just stick with the veg until week 2 of 12-12, mature plants start flowering around 10 to 14 days after flipping 12-12, plants that are not showing sex at 12-12 can take 2 to 3 weeks before they start flowerind depending on age.
how are you finding the DnS nutes now, does everything seem to be working ok