I remember watching a firepower demo at Larkhill, UK., when I was still in Airborne, back in the late '70's, when a R.A.F. Jaguar, and a Harrier, straffed ground targets, including some old APC's and tanks. If I remember right, the tanks were Churchills and late-model Shermans, as used on a number of British A.T. ranges at the time. Both the above aircraft were armed with the 30mm 'Aden' gun, I think the mark5, which was basically a Royal Small Arms Factory development based on the German 30mm MK213 aircraft cannon.
The APC's received multiple hits, with total penetration (they were old Saracens) and the tanks had strike marks, partial penetration on side armour, full penetration on upper hulls, rear plates and engine covers.
The ammunition used at the time was, we were told, a 5/5 load of AP/HE. The HE is a 495 gram round, tungsten carbide, with a MV of approx 2,630 fps, at a cylic rate of 1,200 to 1,400 rpm. The Aden is effective against most targets, except modern MBT's. Given the above, the 20mm would do some damage to the rear of a Sherman, as I believe the MV was higher than the MK213 weapon, although with a lighter grain-weight round (I need to check it out.) but I'm not so sure it would disable the vehicle. I'll see what I can dig out, if I can get mobile; damned arthritis is giving me hell, got ankles and knees like an overweight elephant!