REAR: In order to go 275s in the rear you need to have your fenders rolled. Now remember to roll your fenders, not bend in your fenders. Do not hammer, tap, hit, heat up and bend, or simply put a baseball bat in between and drive back and forth.....you will mess it up. If you want a pro job and no one notice your fenders have been rolled then you go with a special took thats made for rolling. Your rim comes off, tool gets clipped on and fenders gets rolled with no dents or paint chippings. This way no one is able to tell its been rolled. Height of tire will not matter, you may drop it and will have alot of free space in the back. dont worry about the height. So 275/30/20 = no problem. Picture of my car below has 275s in the rear on 19s with coilovers to show you the drop its able to get. So the height doesnt matter.
FRONT: 245/35/20 in the front with your drop should experiance rubbing on the front inner fender cover while taking turns. What will happen is eventually your cover will get worn out and create a hole. You have to option of cutting that part out or remove the entire cover. You also have the option of going with 245/30/20 in the front where it shouldnt rub but you may experience your speedometer to be off about 3 miles.
Thats why its recommended you stick with 19s on these vehicles although upto 22s have been installed before.
For additional information my buddies at Need4Speed would love to help you out. You may contact them toll free 1888.714.7467