I used the Bouchion KD and it works well. I don't think the vacuum and KD (to 2nd gear) from LU will work at same time.
I guess it depends on where you have the LU set. Mine is set to 50mph, so when I stomp on it, vacuum drops, LU and OD cut out and it goes to 3rd gear.
I think kick down from 3rd to 2nd would happen around 30-40mph tops.