Somewhat long-winded addendum:
I think the double duration doesn't require much of an explanation, as the mage armor spell durations are just too short, and bringing their duration in line with the duration of conjuration weapon spells doesn't seem like an OP gamebreaker to me. But the double AR might feel a little overpowered, depending on your priors regarding how much damage reduction a cloth wearing mage is entitled to. Bethesda clearly intended magic heavy builds to stand apart from melee oriented builds. But it is generally accepted that at higher levels the promise of the glass cannon style cloth mage is broken, as magic damage simply does not scale to the same heights that any smithed/enchanted/weapon skill perked Dovahkiin can go.
The simple work around to the squishy mage problem is obvious enough: just give up the enchants on items like the archmage robes in favor of custom enchanted gear on smithed armor that gives max AR and just as good if not better magic cost reduction and mana/mana regen gear. While this path is available, it excludes a whole line of items (i.e. all the cloth gear in the game) from serious consideration as endgame gear for no good reason.
The vanilla armor spells have other deficiencies that seem particularly glaring. The ebonyflesh expert level spell, with three perks spent on mage armor in the Alteration tree, will not even get a cloth wearing mage fifty percent damage reduction. Given the risks and nuisances associated with mage armor spells, one might expect better performance from them, but instead we get lackluster damage reduction and the perpetual risk of getting one shotted at higher levels when the spell expires if we fail to recast it fast enough. And the oakflesh spell is so weak (you get 40 AR, which is the practical equivalent of armoring up with a bottle of grease) that the only justification for casting it is simply as a way to spam for Alteration levels-the protection it offers is too negligible to be worth the effort of casting it on the merits, making it effectively impossible to level Alteration in anything resembling natural gameplay.
It turns out doubling the AR values of those spells tracks a lot better with the DR of equivalently leveled armor characters, which is a coincidence happy enough to make one wonder whether that hadn't been their original design to begin with before someone decided to scale them back. A low level light armor wearer with no perks wearing fur armor (i.e. the worst armor in the game) gets 44 points from a four piece set, plus 100 points from the implicit 25 point AR bonus one gets for wearing any single article of armor. With one point in mage armor at level 30 alteration, with my mod, a mage goes from 80 to 160 with oakflesh, not much different from what one would expect to get out of a low level steel or leather set with a little investment in the armor tree. It's difficult from there on to really assess how comparable the different lines would scale, since that largely depends on decisions individual players make. The main credible gripe a person might make of this mod is that it gets results equivalent to what you can get with smithing and an armor skill in only one skill tree with fewer perks. I might even agree with that. All I can say in my defense is that this mod is an imperfect solution to a problem that shouldn't exist in the first place.
Ebonyflesh with three mage perks will now grant the caster 606 AR, which is about %76 DR, four points away from max DR. I could have tweaked it a bit as this is probably a tad too high even for an expert level spell, but it is worth pointing out that in order to get this result one must still be committed to recasting a spell once every couple of minutes and restricting one's gear to cloth only items to benefit from the mage armor perks, and that an equivalently leveled armor user with a little additional help from smithing is probably already hitting the DR cap by the time an equivalently leveled mage is casting expert level Alteration spells.
And since the cap is generally regarded as being 667 ar (without armor), this armor value makes, for instance, the 50 points of AR one gets from the Lord Stone suddenly much for functional, as the additional DR at the end the the AR curve is much more significant than at the beginning. With ebonyflesh, the lord stone, and an armored amulet like the Amulet of Articulation plus the Diadem of the Savant, a cloth mage can enjoy the benefits of the AR cap while leveling their alteration through to the end of the tree.
Since ebonyflesh brings one so close to the armor rating cap, Dragonhide needed something to differentiate it from the lower level spell variants, and it seemed to me that making it a constant effect that one never needed to recast was the way to go. I would have preferred to have it as a spell that you only had to cast one time, but the creation kit does not allow the constant effect property to be associated with casted spells. As a consequence, anybody who gains access to the Dragonhide spell can benefit from the effect just by reading the book, regardless of their Alteration level and/or magicka investment. While I would have preferred there to be a casting threshold, as well as a utilization of the master casting animation that truly seemed to justify all the dancing and finger waggling, it's still preferable to have the master line of armor spells to give the player something that feels like it justifies all the effort to acquire it in the first place: in this case, the incredible gift of never having to cast another freaking armor spell that goes off like the sound of a truck crashing into a storefront window ever again. And being able to cavort about naked with 80% DR. Hey, you are a mage, after all.