Hull re-inforcements & Armour at the moment primarily increase your total hitpoints (which indirectly reduces penetration chances - after taking the same punishment your hull health is higher which is factored into step 3).
Just a note to be clear - What does Armour mean in this sentence? Bulkheads, or the stat that ships have, and HullReinforcements increase? =]
Somewhat repeating this other poster here ^ but you are referring to the "Armor" stat visible in the Shipyard, correct? If so, does this value indicate your ship's base HP directly, is it modified by other factors, or is it a modifier itself for some hidden base value?
It would definitely be nice to have more hard numbers about things like this, as well as vital stats for things like shields and weapons. How shield HP and regen rate are calculated, base damage values for all the weapons, projectile velocities of all the projectile weapons, etc.
If a lot of these things cannot be successfully determined until the relevant module is installed in a ship (for example, if ship mass, Powerplant capacity, and/or Power Distributor efficiency influence your shield HP and regen rate), then a "Ship/Module Info" panel in the 4 menu would be a good place to put this information. Here we could view our ship's total HP, its current armor values, its shield/weapon stats, max speed, etc. for our current configuration and power allocation (so if we move some pips and open the "Ship/Module Info" panel again, it would update any stats that were altered by power distribution). Putting everything on one screen might get a bit crowded, so perhaps allow us to highlight individual installed modules in order to view in-depth info that is specific to that module (again, with all current modifiers applied).
Making that kind of info readily available would go a long way towards helping equipment discussions be based on a solid foundation, rather than rumor and hearsay.