http://elderscrolls.wikia.com/wiki/Stalhrim_Greatsword <---according to this, if you put a frost enchantment on it, the effect would be increased by 25%. Just like with the perk, "frost enchanter". However, if you have the Frost enchanter perk already, would that stack? Like would frost enchantments added to the sword be 50% stronger than normal base enchantments? Would the sword keep it's 25% frost enchanting effect? For example:

If you have the frost enchanter perk. And put a frost enchantment on say, a steel greatsword. And one on the Stalhrim greatsword while using 2 soul gems of the same level on each of them to hold the enchantment, would the Stalhrim Greatsword's Frost enchantment be 25% greater than the frost enchantment you put on the steel greatsword? Or does the frost enchanter perk nullify this?

I think it does stack. Try it for yourself. Save prior though.