How well would have Chris Byrd been at middleweight?

If you say so.

If that was really the case then you would already know the answers to most of the questions you have been asking here

No. His pro debut would of been at lhw. As I said before you don't have to make the official limit until a belt is on the line.
He started fighting at mw from his 2nd pro fight onwards

And your point is?

It doesn't matter. How many times do I have to tell you that?. As long as he is not abusing his body in between fights then it doesn't matter how often he is fighting in any given year.

Hagler was a mw for his entire career.
Hopkins didn't move up from mw until he was around 36 years old.
Calzaghe didn't move up to lhw until around 36 years of age.
Tarver didn't move up from lhw until he was around 40 years old.

It's the guys who put on massive amounts of fat in between fights that struggle to stay in their natural weight classes

???????????
As I said before, your metabolism won't start slowing down due to age until after 33 years of age.
The only other cause for it to slow down is if you yo-yo in weight, like Tarver and Toney did, but why is that a issue here? This thread is about a hypothetical situation.

Hopkins, Tarver and Calzaghe did it until they were 36 years old.

I don't need to change your opinion. You think what you want to think.

This thread is about a HYPOTHETICAL scenario. Yet for some reason you only want to focus on potential reasons for Byrd to not fight at mw.