You do seem not understand what cost efficiency is. Tacs lose to melee units for cost. A single slugga squad wins against a single Tac squad. A single banshee squad wins against a single Tac squad. Cost efficiency has nothing to do with the maybes - that is tactics.
In, and of themselves, tacs lose to melee. When your opponent stacks them together, however noob and easy that tactic is, it's a valid tactic that transforms their strengths into ability to destroy autoattacking melee on approach. But it does not change the fact that Tacs lose to equivilent cost of melee units.
A blob of guardians or shootas equivalent of Tacs in cost will ALSO obliterate melee on approach, even more so due to well placed grenades and suppression. The only difference is that Guardians or Shootas don't have their durability or upgrades that give them AV. Thus, nerfing Tac melee will NOT change the critical mass concept. All it will do is screw SM in the ass, while leaving the main problem, the durable wall of ranged damage, intact in team games (the only place where Tac massing is an honest problem).
SM tier 1 combat units are inefficient in terms of damage and fielded squad numbers (have you seen the stats on scouts? They are, by far, the freaking worst unit in the game without upgrades); that's the trade off they get for whatever advantages they have, such as their low unit squad count and higher for cost HP.
Let me put it this way: Scouts lose to almost every unit they encounter on the field in tier 1 while capping, unless you get lucky with an inattentive opponent or with garrisoning. Tacs will win only against single enemy ranged squads or some melee heroes. In effect, to win tier 1 engagements SM *have* to blob or rely on an inferior opponent. By weakening Tac melee you remove heroes from that list. Your idea of balance is for SM to lose basically 80% of tier 1 match ups unless they blob, face an idiot, or get lucky with a building?