I've been tested at 238w and 234w when I was 72kg. Looking back through some zwift files (I have a Tacx Neo), I've done 230w for over 20mins in some training sessions when I was 66-68kg. I once did a FTP test on zwift on a cheaper Tacx, and it claimed I'd done 306w for 20min. I definitely didn't. I could typically do okay (win sprints, finish top 10) in 3w/kg events on Zwift, but 3.5w/kg were really hard.
Looking back at 10 months leading up to LEL2017 I spent training really hard (up to 20hrs a week, loads of turbo time, following a structured training programme, thinking about nutrition all the time), I learned that I don't improve much. I have really quick mates who ride less, in a less focused way, and all that training just meant I could mostly keep up with them where they would normally drop me (ie the short, sprinty stuff). I don't think I became a significantly stronger audaxer.
I find losing weight easier than gaining power. Having been down at 65kg (bmi 17.x), there's a point at which the losses from power outweigh the gains for me, but it's way skinnier than I thought it would be. I could float up hills but a headwind would kill me. In the end, my chest doc told me I was not carrying enough fat reserves if I were to fall ill with my lung condition.
I don't think I will ever train like it again. It took too much focus, and didn't make enough difference. Also, when you are over-trained, it makes you really shit. I spent much of that time feeling ruined (with some really horrible symptoms). Occasionally, I was on form when I wanted to be (DKTTR/BCM/LEL), but a few times I was in a bad way when I started an event, and it only got worse (MilleP, 3 coasts).
I still like training with power / HRSS, and admire people looking for change in FTP. Back to your question, having been at the sharp end of normal audax, I think most probably have FTPw/kg around 3.5ish. The people coming in from racing or 24hr+ racing tend to be younger, and I could never keep up with them.
I would like to think with a bit of focus, I could get back to 2.7FTPw/kg. I don't see myself getting back to the far side of 3w/kg any time soon.