Roman helmets: Imperial Gallic/Italic and Ridge - comparisons and sources

[Crispianus]

"It would be totally possible to throw some models of a Ridged helmet and a suitable Italic/Gallic counterpart into a FEM program, maybe Solidworks etc, and simulate a Impact force on multiple points of either helmet, but I think it is a totally fair assumption that the ridged helmet would lose out at some point (who knows though, it may be well above any impact force a human could produce reasonably). It is not necessary to test this case."

Well that really is the crux of the matter, all things being equal, the probem is they arn't.
At the time as we would understand it there was no standardisation (this doesn't actually occur untill well into the industrial age) in material thicknesses, composition, strengths/ hardness / toughness etc, surviving helmets whether one piece or multi piece vary by degrees, so any armorer would be going with physical results alone and as a result different helmets would give varying protection independent of construction.......
Any modern tests would have to take into account the lowest and the highest common denominator in surviving helmets, and its likely some one piece and some multi piece examples would fail the acid test, however as long as they are:

"above any impact force a human could produce reasonably"  They would prove to be an effective protection.

EG: the one piece helmets worn during Trajans campaign required strengthening (some were done in the field), later helmets had this additional protection as standard.

Ridge helmets are said to be based on Iron Persian examples and its quite possible considering the above that these proved equal or superior at less cost/ease of manufacture hence their adoption, materials are likely the main cost factor at the time since once you have plate, to produce a basic helmet of any type takes a similar amount of work dependant on its detail complexity, manufacturing material by hand (or more likely with trip hammers) is very time consuming with the iron needing several processes, remember that charcoal is the main source of fuel and you'd need a lot of it as well likely more for one piece helmets.

larger pieces of hand produced iron plate are more likely to have faults resulting in failure when producing a one piece helmet, though this material could be recycled in multi piece types or iron scales etc, so nothing except the labour which would likely be done by slaves would actually go to waste.

In short if you want to make a helmet to protect you against more force you increase the thickness or quality of the materials or add additional external protection, construction plays a part as does shape, a conical helmet as in the original Persian style(and some ridge helmets) would prove more effective to many attacks then a dome whether one piece or multi piece, but the effective protection is entirely dependant upon the thickness and quality of the material used and any heat treatment or other process that was known at the time, such as Carborisation.

Unfortunatly we have far too little material detail on most surviving helmets (there are a lot of reasons for this), so untill more is available this will simply remain speculation..... but its probably a good idea to look up manufacturing and blacksmithing techniques for iron from the period, especially where this includes studies on surviving items of all types.

Eg: Phosphorus if a high enough content is present in the ore can have a significant effect on the hardness of the iron, making it unnecassary to use carborisation although the two combined can produce significant results, on the other hand one piece objects usually require a less hard material for ease of hand working..