Overheating / loss of power

So the mechanic that I had do my annual left a radiator hose loose which I did not know about until I was 2500 feet and noticed the temperature gauge was buried. I immediately flew back to the field and made it. I had‘the heads sent off for hardness testing and one of them was compromised and replaced. Now that the engine is back together seems to be making less power than it was . So I have two questions.

1. does anyone have any suggestions on what else might have been damaged to cause a loss of power?

2. this one might seem like a dumb question but if my rpm is the same is it possible to have reduced Horsepower at that same RPM?  Like for example… my airplane used to run 92 kn and level flight at 5200 RPM. 

Now it runs around 82 kn at 5200 RPM In level flight. Does this indicate a loss of horsepower or if the RPM is the same should the airspeed be the same?  I’m a little bit confused and I hope I’m explaining this correctly. Basically I want to know if it is possible to have less horsepower and therefore less airspeed if my RPMs are the same at 5200?

 The reason I ask is that I’m not 100% sure it’s a loss of power because we did replace the prop although we put the exact same prop back on and the degrees are supposedly exactly as they were. it was double checked by two different mechanics and confirmed it was exactly the same setting as the prop that came off.  So the prop setting should be the same. And therefore my speed should be the same unless I’ve lost horsepower. But again I’m not sure how that works if it’s even possible to have less horsepower but still be turning the same RPM?

thank you for your help and have a great day!

Dylan