Martial Law (ML) Combat is basically the core combat system but with location specific tables and damage adjustments to armor (in addition to the DB). So, this system is more deadly for unarmored people like Wizards, Swashbuckler or Martial Artists and less deadly then the core system for any kind of tanks. The book also contains a couple of other things as RWW already mentioned.
Hack & Slash (H&S) is a bit more different, because it removes the damage caps from the tables, which makes it quite deadly. However we use it since 2 years, and I was unable to kill a single player (thanks to fate points). It is a One-Roll system (as the core system), so I have absolutely no clue what RWW meant with ?a lot of dice rolls?. The bad thing with the system is that the description of the criticals is sometimes a bit bland compared to the core system or ML and because of the removed damage cap scaling of elemental spells is rather useless. But regardless, I think it is the fastest system and pretty usable. It also has an optional rule for hit locations (which modifies the damage), but I never used it.
The HB#11 Combat System is borrowed by Rolemaster Express and is very similar to the RM/MERP combat system. It has the nicest description of criticals and seems to be the deadliest one. However, I haven?t tested it, because it needs two rolls, one for to-hit and a second one for the critical (as Arms Law). We moved from RM to HARP to speed up the rolls and the play itself, so this seemed for us to be a way back.