Is it actually possible to put level 50 mods on a level 9 orange armor piece?
Also, I'm willing to bet the base armor value does increase towards the endgame line of oranges, even if the above is allowed.
Orange gear has no base stats. The armoring you put in it determines its armor value. The barrel you put in a gun determines its base weapon damage.
So yes, you can put level 50 mods in a level 9 orange piece of armor, making it a level 50 piece of armor with level 9 looks. That is the very purpose of orange gear.