I'm not so sure about this. My vatf seems to growing at a good clip still now at 15-16". I'm thinking you will probably be better off going 8x4. They are incredibly fast fish so the more room the better.
I grew my old ATF in a 55 gallon until they were 6". Easier to pellet train in a smaller tank. After they hit 6" I moved them to the 180 and a year later was 11".