Tie broken. Do it.

I am running 18/20 stagger with no problems whatsoever, and when you think about it, it is all about maintaining the same overall size of the wheel + tire package. You will obviously give up some sidewall to accomodate the 20" in the rear, but you will have plenty left where you will not be driving on rubber bands. I used a 275/30-20 and am VERY close to the correct speed at ~60MPH, maybe a half mile an hour off, but since every tire manufacturer uses slightly different sizes for the same tire, it will depend on manufacturer as well. Good luck, shoot me a PM if you need anything further.

John

PS: I think the loop, especially lowered (like mine) is the cats meow...