Don't underestimate sentry+dig in. You can over come up to 3k CS 
Its important to understand what both of those do, and what they modify.
Sentries provides a bonus to CS. You could calculate what the precise bonus is if you were patient enough. I'm actually uncertain what it is. Digging in reduces hits taken, not hits given. This would only affect apparent CS for the calculation when determining how many losses they take, not how much damage they inflict. In effect sentries gives a flat multiplicative modifier of CS, digging in simply delays a retreat.
In the context of what we're talking about with changing the combat code, only the effects of digging in on lengthening combat sequences is new. I'm actually very curious about using to flanks setup with units in They Shall Not Pass and piling everything else into the middle. This could in theory drag combat out to five turns (3 turns in my analysis' parlance) on the flank and the middle would just absolutely crush the other side. It would probably eat up 20% of total CS, and might be the only way a smaller army can defeat a larger one. Obviously however, the commander of a larger army could just send 70 15 15 and the smaller guy still wouldn't be able to beat him at 50/50/0 or even with 10/80/10. In other words the more disciplined army with greater leadership skill in game would win.
