From: east coast, usa
For breakthroughs, higher attack values, more action points and/or more units with double attack, I guess...
Well, I can see that you are into it, so ...
You are correct in what you say above, but there are a few other considerations:
Your spearheads rely on vulnerable HQ's being nearby to provide supply. Since HQ's can't hold the line, this means that any breakthrough needs to be three hexes wide.
For an encirclement to be successful, all supply generating locations within the pocket need to be captured. Otherwise, the surrounded force can replace to some extent, which extends their survival and prolongs the encircling force's exposure. This isn't practical on a map where supply generating locations are placed every two-three hexes. Additionally, HQ's within a pocket need no supply at all to provide a level or replacement.
Old boardgames that use similar mechanics also include rules that unsupplied units cannot move or attack, or both. In SC3 there are reduced capabilities, but surrounded units can still move and attack, which makes it more difficult to sustain a successful encirclement.
That's enough for now