A 33x10.5 has been used a lot. With a 4.5" backspace on the 8" wheel it would be pretty okay.
Personal suggestion tough, a 33" tire on a Z85 will rob a tremendous amount of power. I am between 14/15 mpg with 31.6" tires. I can't imagine even larger. The 33" tire is over 15% increased in diameter. Z85's sit lower than Z71's. Overdrive on a grade if any significance is pretty much useless making it difficult to use cruise control on a long drive. The hills are always forcing it to kick down.
Without considering the mileage, the vehicle feels so sluggish too when accelerating from the line like at green lights and stop signs.
Here is a pic of my truck with 265/75-16 (31.6") on a Z85 sitting at 24" hub/fender. I've installed 1.5" shackles and I sit level. If you haven't lifted that high, no way will you fit 33's. I wish I'd gone with 31's to be honest.
http://coloradofans.com/forums/members/69977-08canyon-albums-random-picture19394-driver-side.jpg