Well as an electrical guy, I always think the problem is in my arena... so here are just a couple ideas.

How did you wire up for knock sensor, that will pull timing and you will feel a power loss.

I think an O2 sensor going bad would also cause a perceived power loss, so would fouling plugs. Check grounds.

What does everyone think. The thing is, when I troubleshoot, I start with "what was the last change that could have affected things" With a swap, it's easy, cause there are a lot of wiring changes.

Now, I'm just discussing so don't flame me. But as far as leakdown/compression testing, there is no way it will hurt, and it's great information. But I don't specifically see the cause that would produce that effect.

-PHiZ