higher compression ratio and stock twins

I saw an article in a uk import magazin that a guy changed the enginge compression ratio to 8,9:1 together with upgraded stock twins. he but down 500 crank-bhp at 1,1 bar boost
what is the benefit in changing the compression ratio to 8,9:1?
I know that the a/f mix volume will be less. Is that means that the stock twins will be more efficent?
I saw that the porsche 996 turbo/gt2 has 9,4:1.