I`m very skeptical of the abilities of Gau-8 to destroy modern tanks, be it merkava or another one, as they are much better armored than say a T-55 or 62. Note that I say destroy not damage. Damage it certainly will do, to targeting instruments, tracks, etc.
The cannon is usually used in short burst fire during a strafe run. Even the high rate of fire cannot guarantee numerous hits at the same impact point. The accuracy Accipiter mentioned on the first page may look impressive, but so far I haven`t seen a video of A-10 practice fire that proves that number. On the contrary, the hits look quite dispersed, which makes me believe 80% in 6 m from 1200 m is a bit exagerated or maybe it shows the accuracy in perfect conditions.