The largest tyres on the W209 CLK as standard were on the CLK55AMG, sizes were Front 7.5J - 225/40x18, Rear 8.5J - 255/35x18,
if that's any help.
The 285's on the rear are 4.6% out compared to the standard rolling circumference. 285/30x18 would be a better match, but you would need wheels 9.5 to11J, not sure the CLK has that much space under the arches.
Russ