The 20 pdr APCBC round (m.v. 1020 m/sec) could penetrate 210mm of armour while the APDS round could penetrate 300mm (m.v. 1465 m/sec). There was also HE and canister ammunition available.
The NATO 105mm L7 was developed because of issues the 84mm and 90mm guns had with dealing with the T-55 at longer ranges and oblique angles due to its sloped/rounded armour. Depending on the ammunition used and date (improvements by all sides to ammunition), the L7 could penetrate between 350mm and 500mm of armour at 1000m.
Even so, the L7 soon had its own problems by the early 70s when the WP began to deploy the T-64 and then the T-72. Even the latest APFSDS rounds would struggle against the layered steel/aluminium/steel armour (more than 500mm in the turret front). Even Chieftain was threatened by 1973. Thus the drive for 120mm smoothbore found on modern MBTs.
Back to the T-55. If CH has an AF of 8 (square or not) for this tank it is not in the least bit accurate. As noted above, by the mid 50s the US 90mm and British 84mm would have struggled with long range penetration vs the turret (ranges >1000m). Then again, in western Germany the average LOS was about 800m (somewhate further in the north) and was often closer to 500m (ie: Fulda Gap). Note that we don't often see even 500m of open terrain in an ASL scenario unless one side is on a hill.
One should also consider that most tanks knocked out were penetrated from the side, not the front. ASL does not model this aspect of tactical warfare very well due to the overly cluttered, narrow maps and small size of the scenarios. Besides, most ASL maps are more representative of mixed village/woodlands than actual European rural terrain.