Band Camp statements: To state "But this one time I saw X doing X... so that justifies X" Refers to the Band camp statement.
Coined by Maxwell
The armor rating of a piece of armor is applicable to both, physical resist and spell resist. In your example, the helmet with 2700 armor rating would give you 2700 spell resist and 2700 physical resist.
If you craft it with reinforced trait, the armor rating gets increased by a percentage (depending on item quality), for sake of simplicity let's say 10%. Thus, this helm, with a base armor rating of 2700 and reinforced, would than have an armor rating of 2970. This is again valid for both, spell and physical resistance.
If you craft that helm with nirnhoned trait, only the spell resist contribution gets increased. So this helm will still have an armor rating of 2700, provide 2700 physical resistance but now (and since the percentage on nirnhoned is higher than reinforced, e.g. 20%) 3240 spell resistance. By crafting this helm with the nirnhoned trait you increase you spell resist by an additional 540 spell resist.
Considering that ~33000 spell resist are needed to achieve 50% spell mitigation (reduction of incoming spell damage), these 540 correspond to approximately 0.8% less spell damage taken.
As to weapons with the nirnhoned trait, these still work the known way from before the patch: If you apply nirnhoned to a weapon, you gain 12% spell resistance at gold quality. These 12% are calculated from your overall equipment provided spell resistance, hence nirnhoned weapons are still a very good choice in reducing spell damage taken, whereas armor pieces generally are only worth the effort if they are heavy armor and chest (highest armor rating) or the intermediate pieces (helm, legs, shoulders, botts, which all have the same armor rating). For gloves and belts, nirnhoned is completely useless.