My avaraged speed unit is between 470 and 500 speed, so I prefer helpers to be around that speed as well, to keep the speed of the whole group relatively even for those maps that require it.
For my regular unit, it doesn't matter much, the main squad is 887 speed, while the second is 742 speed, so they're usually leading the other squads anyway, and generally very capable of clearing an entire map and some alternative routes. My second squad might drop to 590 speed soon though, in which case a fast helper might become useful.