Have you thought about having them re-route the plumbing through the attic? While I’m not fond of doing this because of the potential damaged caused by leaks in the attic, it might be less expensive than digging up the existing pipes and replacing them through the slab. Most plumbers won’t do it that way, anyway.
If you’re wondering about how to bring the pipes from the attic to the island, you can run them through support posts (functional or decorative) that go from the island to the ceiling. You won’t get the “floating island” effect, but I think it looks at least as good (IMO, it makes the island look more substantial).
