So the response of the ECU to a bad knock sensor is to max out the fuel trim. What that means is it runs the engine as lean as possible which would definitely account for the lost power. Looks like knock sensors run about $100 each, harnesses are around $45 and labor will probably be a few hundred.
Btw there's a known issue with rusting of the knock sensors so when they're replaced make sure they follow the TSB and put in the silicone dam to stop future water intrusion which should keep you from needing to do the repair again in the future.