Item improvement has nothing to do with the Enchantment effect. With the exception of an item with the Infused Trait that has a Glyph applied. How much the item is improved determines how much the enchantment is boosted in that one case.

Now, something like a Purple Ring will have some better base stats to start with, and that can be a good thing. But the Glyphs you apply will give the same on a white ring as a purple ring.