Customized Weapons

I know this is a noob question, but its been bothering me for a while. When you costomize a 15^50 weapon, does the 20% replace it, or add on to it? Because if it replaces it, why not just costomize weapons with +5 energy and get both the +5energy and the 20%?

it adds to it. with a 15^50 inscription, that's +35% damage.

Does it just add straight on or is it some weird math thing like with crit strike stacking?

It adds by percent. It's not 35%. It's 1.15 x 1.2 = 1.38, 38% more damage.