That's a good point ay, if your fronts are running fine with a 50mm drop and 616mm diameter tyres, should be able to run about 640mm diameter tyres at least without hassles, which is well above the legal limit already
And holy crap, 20's???? Had to see that on his ride thread haha, looks great! And it's on sportlines!!! He reckons they're either 225 or 235 with 30 aspect ratio, so say best case with 225/30/R20 and he has the 30mm drop on the rears... *get out the calculator* ... 643mm diameter tyres, wouldn't want to go any more than that then without serious guard work!