Not sure how far in the hole the slugs were on a 66, But....
I had a non molested hp 67 440. The pistons were .065 in the hole IIRC. Then the .020 gasket and some 516's that cc'd around 79.
Comp calculator gives that just over 10:1
Switch to 452's and a felpro gasket, and you're down to just under 9:1.
So a full point of compression, but you did gain a bigger exhaust valve and bigger runners in the heads. So there is a debate unto itself there.
Realistically, the gains in a closed head are only seen when quench is present. With pistons that far in the hole, you have no quench.
A full point of compression should only cost you 3 to 4 % but I think there are some other variables here.
Some say that a closed head develops some swirl on the intake charge, that improves atomisaton.
So it could be that with the open heads, it became slugish down low due to poor atomisation of the fuel charge. Add on the slight overall power loss because of the comp loss, and it may be noticable in the pants dyno.