Unit stats and their effects

This may be a stupid question (and I have a gut feeling it is), but does altering a units stats, specifically the armor and particularly shield stats, have bearing on it's battle performance. I don't mean any simply change, so let me clarify:
As it stands (vanilla version of the game), all infantry have either a 2 or 5 shield, and all cavalry have a 4 shield (RSC had 2 in 1.1 and earlier, as I believe did certain militia cavalry). So what I am asking is whether or not changing a shield value to 3 or 4 for infantry and higher or lower for cavalry with actually have an effect, in-battle, on a units ability to shield themselves from arrows, etc.
This would also make it easier for me as so far I have been reluctant to change shield values, simply for this reason, to ensure a more complete, balanced game.

Again probably a stupid question, but I have to know for certain (I'll probably go and play right now to test it out, but a quick answer from a more experienced modder would be painless). Thanks.

Shields are like a seperate armour factor that only points towards the front and the left.

I have personally tweaked a lot of shields. Iberian Infantry gets a 3 shield as does many pikemen (not Levies though), most cavalry has dropped to 3 as well and those big wicker shields of the east are now only 4. Meanwhile I have upped the late scutum to 6 (but I have taken a point of armour from the legionaries). That fits my view of the things. And it doesn't destabalize the game.

I've been planning similar for the shields. There are currently 2's, 4's, and 5's. I already gave the Iberians at 3 since the caetra deserves that. Some of the smallest peltast/slinger shields probably deserve only a 2 as given.

The early principes/hastati probably deserve their 5 shields, although the shield shown in the game is probably a bit better than what they had at the time. A 6 or more likely a 7 for the later legionary scutum seems right.

You will find the chosen swordsman getting a 5 for their small round shield...that is clearly wrong. Their description doesn't match their graphics, but since I'm $3,000 short of the software needed to change this, I am going to switch them to a 3 instead of a 5. Regular swordsman are getting a 5 from a mid size round shield. I'm probably going to give them a 4.

The hoplon aspis I will probably leave at a 5 although I might give the metal covered ones a 6.

I am giving the mid size round cav shields a 4 and the smaller ones a 3.

Large ovals = 5, mid size ovals = 4.

I agree that wicker shields should not exceed 4 for the largest.

While I agree that certain barbarian shields don't fit I'm very vary of changing them to the worse. They are after all weak already (the factions), weaking them more is not good, and adding the excess to defensive capability does not make up for it. I have though given the Woad Warriors a 3 shield.

And can you guys say for certain that changing the shield values has actually has the intended effect in a battle?

Very much so.

We can of course not see much change in the fight itself, but when you pause and rightclick on an affected unit the stats will show up in its altered form.

Well that is what I want to know, whether or not it has any relative effect in battle. I know the stats themselves change, but the shield values (at least to me) seem to be fixed, and I am wondering CA did this for a reason? Maybe a 4 shield is no different in defense of missles as a five shield because a unit is assumed to have one of only a couple of shield options. I want to know whether or not it has a definite impact on the unit in battle, not just a change in stats on the unit card and description.

Well you can mod two units to be the same, say Scutarii and Hastati but give one of them a better shield (Scu a 3 shield and Hastati a 5 shield). Then have them duke it out in a couple of Custom Battles. Then give the Scu a 7 shield and test again. I'm quite sure you will see that the Hastati will win the first battles and lose the others.