I have 32" tires on my D5 right now, no rubbing, no cutting, no lifting. 33" will fit without mods but will rub at full lock at access height - i.e., crouched down driving in a parking garage.
EDIT: For the record, my 275/55-20 AT's are 31.9" tall and 265/65-20 KO2's are 32.6" tall.
Go 18"...