High or low octane gas, is there a difference?

[Spline64]

Hello,

Hoping you can all chime in on this one. Want more opinions than what I have read and heard so far.

Have a '73 454 with mild cam (no duration specs sorry), headers, edelbrock performer 2.0 intake and a edelbrock 1405 600cfm carb. Otherwise a stock motor.

Engine had about 10,000 miles on it after rebuild when I put it in my truck. I have been running 91 octane in it because I basically didn't know better. Thought that I would get better performance that way but I guess that isn't quite true.
I'm at 4,750 ft. elevation here and it was proposed to me that I'm basically wasting my money, 87 would perform the exact same due to the lack of oxygen. I had ran a tank of 87 before and thought I noticed a loss of power and rougher idle, but that could have been just me. I did not notice any pinging at all. Timing seems to like to be 10 deg. advanced (initial)

I'm going to fill up with 87 this weekend and really pay attention to performance, smooth idle, ect.

So does octane actually help with horsepower? Or should I go with a lower octane and tune the carb accordingly?

Thanks,
Pat