Well, to be fair if someone paid the cost to put that on Military Armor, I don't see a problem with the magic increasing the Hardened value. The cost and difficulty to obtain it all would make it rare enough as-is.
At first I would agree, but having Hardened Armor into the thirties (easy with a set of mil-spec and a moderately high Force Armor Focus, assuming 3 points of Hardened per point of Force), gets a little too crazy from my perspective. With Heavy Mil-Spec w/ Helmet, you're getting 32 points of Hardened Armor with it being enchanted at Force 3. Assuming an equal Restricted Rating to Power/Weapon foci, Force 3 is only 12R, which is easy compared to the armor itself. Light Mil-Spec w/ Helmet would hit 30 at Force 4, so 16 Availability for both Focus and Armor. Assuming a tied cost with Weapon Foci, and that's 21,000 nuyen and 28,000 respectively, so it only surpasses the armor's inherent value by a little (or the armor surpasses it a bit if you have fancy upgrades like a chem-seal).
Also, though this doesn't factor into Character Creation, the Object Resistance of the modern Mil-Spec Armor, with all the bells and whistles, would probably make it tough to enchant in the first place. That could be a possible limit, I suppose.
My big concern is the fact that you can ignore most everything. Even the super-tank-troll with full mil-spec, along with all the bone lacing and dermal armor he could get, is only getting 23 Hardened, and 50-60 soak. A high enough hit could hurt him, provided he flubs his soak roll. Allowing Hardened Armor to stack takes that 'high enough hit' and makes it near impossible. I mean, a guy with a Thunderstruck or Panther XXL sniping him might be able to damage that guy (23 Hardened, -8AP to 15, 15P damage as the base, or -6 AP with 17P as the base, respectively), provided he can shoot the guy; same goes for high-explosive rockets/missiles if they get a direct hit; he's ignoring grenades and frag missiles. At that point, the only guaranteed penetration is from a anti-vehicle missile, what with 24P, -4AP (he's not a vehicle, so it uses the lower AP value). Once you hit 29 Hardened, you're immune to practically every weapon (Actually, unless I'm missing something, it IS every weapon) in the book; it'd take a dedicated combat mage to hurt you, what with the AP value of combat spells cast high enough. It'd take a Force 15 Indirect spell to supersede the anti-vehicle missile in damage, and hit the 29 Hardened with a good chance of success. Direct Mana Spells would be the only realistic way of touching him, and you'd think he'd have a high Willpower if somebody put him in that suit.