That is one deal where the only answer is more compression--IMO I would drive that good running combo forever and keep that $$ in your jeans but--if you want it to pep up do it right and swap the pistons leave the rest as is
Milling the heads just can't make up for a piston that is so far down in the hole that is like water in a well
Just drive it or.... put a 125 shot of NOS on it and for $400 and two hours you have done the most with the least and made it a real Hot Rod that drives easy otherwise
Perfect candidate for a 125 shot I mean Perfect!