I'd say it depends on your SG and troupe but in my experience the answer would be "yes" if you play with easy people, and "no" if with some historical maniac.
Let me instead turn the question around. What are the advantages for treating a suit of armor as a single item for enchantment purposes? What are the disadvantages? I don't believe that there is a canonical answer for this, but canon is rather large and, unfortunately, contradictory in a lot of places.
personally I'd say if you can enhchant a suit of armor in real terms (base metal: 5 pawns of vis, human sized is x5 modifier, 25 pawns of vis requires MT:13) then yes you can enchant a suit of armor. Sure it is a collection of component parts, but so is a wagon (wheels, seats, planking...) and it can still be considered a single thing.
Or you could choose leather and drop it to 10 pawns of vis, MT:5.
In our saga that's each enchanter's choice. You can enchant whatever you want: a breatsplate, a couple of gauntlets, each gauntlet, the whole armor... even one single ring in a chain mail (the same goes for anything else: when enchanting a sword you can enchant the whole weapon or just the handle, and then leave the blade mundane and being able to penetrate through magical resistance without problem).
The only two problems one uses to face when he goes for the full enchanted suit is that the whole armor must remain complete for it to work and that it requires bigger ammounts of vis to open. The problem with the piece-by-piece method are that they require more time (if you make them enchantment devices, then you will spend a lot of seasons opening pieces for enchantment. If they are lesser devices the only problem is that they would be less powerful but they can be useful anyway) and that to achieve some effects you must go for higher level effects. For example if you have a a piece-by-piece enchanted armor and you want to strenghen it with a Doublet of Impenetrable Silk effect, then it could be argued that you must raise it from T:Ind. and R:Personal. to T:Group and R:Touch to affect the whole armor, adding 15 levels to the enchantment, but having the advantage that you can replace pieces of your armor without problem and still getting the benefit.
There are also a few ways to enchant things with lower MT scores. The ones that jump to mind are Verditius Magic, Verditius Elder Runes, Philosophic Alchemy, and Imbued with the Spirit of Vim.
I'm not convinced you are correct using x5 for a metal suit of armor.
A metal suit of armor is not as big as a human. It's easiliy transportable, and I would use a x3 mostly.
Not because of the enchantement itself (there are too many way around, like those evocated by Callen) but because of the basic number of vis pawns it would allow. A word against fire, metal is really effective on an armor and less hard to deal with than if you ward yourself against metal.
Looking at the table on page 97 of the ArM5 core rules, a shield or a cloak is large x4, and a suit of armour is bigger than that. On the other hand, a skeleton is x4 and that's only a bit smaller than a human. I think following the guidelines from that table, x4 is probably right.
If you look at the Reagents section of A&A, the Alchemical Steel reagent (on page 76) affects 50 cubic inches of metal (Size +0, complex matter) and states that 6 doses of said reagent are required to treat a partial suit of armour.
So, 6x50 = 300 cubic inches. The Volume Conversion Table on page (also on page 76) lists 500 cubic inches as being a Medium object on the Material & Size Tables in the core book, the same size category as a tunic.
Sizes in the corebook are weird: rooms and human bodies are both x5, but you can store several bodies into a room.
So I guess room is also the size of a body because it's mostly empty space. As an armor is empty on the inside too (unless you enchant it attached to a human body or anything else, and that would be weird), I guess full suits of armor should be x4 and partial suits x3.
It looks to me in the core book that x5 is anything human sized or larger, including a room or a suit of armor (I could see where a partial would be x4), capping off instead of having a x6 category. Obviously this changes in certain supplements...