First of all, you're the one that's wrong, but I'll cover that in a moment. Second of all, here's my comment regarding Moore's Law is Dead, as you're assuming shit you shouldn't:
Now, for the Compute Unit conundrum. Even if you are repurposing a Compute Unit, it does have its customization for it to be an Hardware Controller for audio. You don't just magically say "You shall henceforth be known as Tempest Engine" and boom, its done. That said, here's how you're wrong
Scenario 1: In this image, all Compute Units are ok and ready to rock. This means that your theory would work, as you have 40 CU's to play with and you can simply repurpose one of them. However, you need to actually say "only the Middle Left Square can be the Tempest Engine" because TE has its own set of customizations.
Scenario 2: Now, you have 2 Cu's that are disabled due to one of them being faulty. Your PCB is still peachy, because you only need 6 out of these 10, and you can still have one Tempest Engine in the middle left. Cool beans!
Scenario 3: Woops, we have problems! Now, even though you have 8 out of 10 CUs, so 4 pairs are still cool, but your whole PCB is garbage because your TE designated CU is faulty or part of a faulty pair. So we guess we just recycle it and pray the next one is right!
What you seem to not understand is that the TE is a separate unit that is not part of the 40CUs, even though its based on one. It's still in your whole APU, but as a separate hardware unit.