Mana cost is a copieable value, therefore the converted mana cost of a spell copy is the same as the cc of the original.

Does a copy of a spell have a CMC?

Does a clone also copy CMC? Technically, no, but it copies mana cost which CMC is usually derived from. If CMC was copiable, a Clone copying the back face of a DFC would have its CMC, but because it doesn't the Clone's CMC is 0.

What is the CMC of copied spells?

Since the CMC of the copies is 9, you'll have to choose 9 as the value for X if you want to play Spell Burst.