OPA still reflects how much offence a given province can run because acres (or accurately, space) is still the hard cap. Nw isn't a cap on power, although you are right of course that nw efficiency will be more linked to gains next age. But that's why it is a trade off. Being nw efficient might help you get wars with people at a similar offence level, but if you end up in a war with someone at a similar acre level (rather than nw level), then you may have less offence than them even though you have better opnw. Your gains would be impacted but potentially you could make more attacks (lots of assumptions here including whether all generals are being utilized in each scenario).

I'm not necessarily suggesting your chart should actually be based on OPA, but just pointing out that as things stand with the proposed changes there seems to be a trade off between power (opa) and nw efficiency/gains (opnw).