From what I have read 40watt should do it but not a lot to spare.
Things you likely already know;
1) Mirror alignment not perfect reduces power substantially.
2) Same with focal length
3) Moving to fast.
4) CO2 lasers lose power over time and use.
5) Dirty reflective and focal lens.
Hope you get it worked out, I know how frustrating that is, I still cannot get my 130watt to cut 6mm but making headway.